Implement an approximation of Linux MADV_DONTNEED semantics.
Linux MADV_DONTNEED is not advisory: it has side effects for anonymous memory, and some system software depends on that. In particular, MADV_DONTNEED causes anonymous pages to be discarded. If the mapping is a private mapping of a named object then subsequent faults are to repopulate the range from that object, otherwise pages will be zero-filled. For mappings of non-anonymous objects, Linux MADV_DONTNEED can be implemented in the same way as our MADV_DONTNEED. This implementation differs from Linux semantics in its handling of private mappings, inherited through fork(), of non-anonymous objects. After applying MADV_DONTNEED, subsequent faults will repopulate the mapping from the parent object rather than the root of the shadow chain. PR: 230160 Reviewed by: alc, kib Sponsored by: The FreeBSD Foundation MFC after: 2 weeks Differential Revision: https://reviews.freebsd.org/D25330
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parent
701267ad19
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Notes:
svn2git
2020-12-20 02:59:44 +00:00
svn path=/head/; revision=362631
@ -38,9 +38,11 @@ __FBSDID("$FreeBSD$");
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#include <sys/file.h>
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#include <sys/file.h>
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#include <sys/imgact.h>
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#include <sys/imgact.h>
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#include <sys/ktr.h>
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#include <sys/ktr.h>
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#include <sys/lock.h>
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#include <sys/mman.h>
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#include <sys/mman.h>
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#include <sys/proc.h>
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#include <sys/proc.h>
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#include <sys/resourcevar.h>
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#include <sys/resourcevar.h>
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#include <sys/rwlock.h>
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#include <sys/syscallsubr.h>
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#include <sys/syscallsubr.h>
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#include <sys/sysent.h>
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#include <sys/sysent.h>
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#include <sys/sysproto.h>
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#include <sys/sysproto.h>
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@ -48,6 +50,7 @@ __FBSDID("$FreeBSD$");
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#include <vm/pmap.h>
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#include <vm/pmap.h>
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#include <vm/vm_extern.h>
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#include <vm/vm_extern.h>
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#include <vm/vm_map.h>
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#include <vm/vm_map.h>
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#include <vm/vm_object.h>
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#include <compat/linux/linux_emul.h>
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#include <compat/linux/linux_emul.h>
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#include <compat/linux/linux_mmap.h>
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#include <compat/linux/linux_mmap.h>
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@ -242,6 +245,98 @@ linux_mprotect_common(struct thread *td, uintptr_t addr, size_t len, int prot)
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return (kern_mprotect(td, addr, len, prot));
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return (kern_mprotect(td, addr, len, prot));
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}
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}
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/*
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* Implement Linux madvise(MADV_DONTNEED), which has unusual semantics: for
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* anonymous memory, pages in the range are immediately discarded.
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*/
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static int
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linux_madvise_dontneed(struct thread *td, vm_offset_t start, vm_offset_t end)
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{
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vm_map_t map;
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vm_map_entry_t entry;
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vm_object_t backing_object, object;
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vm_offset_t estart, eend;
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vm_pindex_t pstart, pend;
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int error;
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map = &td->td_proc->p_vmspace->vm_map;
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if (!vm_map_range_valid(map, start, end))
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return (EINVAL);
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start = trunc_page(start);
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end = round_page(end);
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error = 0;
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vm_map_lock_read(map);
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if (!vm_map_lookup_entry(map, start, &entry))
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entry = vm_map_entry_succ(entry);
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for (; entry->start < end; entry = vm_map_entry_succ(entry)) {
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if ((entry->eflags & MAP_ENTRY_IS_SUB_MAP) != 0)
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continue;
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if (entry->wired_count != 0) {
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error = EINVAL;
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break;
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}
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object = entry->object.vm_object;
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if (object == NULL)
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continue;
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pstart = OFF_TO_IDX(entry->offset);
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if (start > entry->start) {
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pstart += atop(start - entry->start);
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estart = start;
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} else {
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estart = entry->start;
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}
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pend = OFF_TO_IDX(entry->offset) +
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atop(entry->end - entry->start);
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if (entry->end > end) {
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pend -= atop(entry->end - end);
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eend = end;
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} else {
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eend = entry->end;
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}
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if ((object->flags & (OBJ_ANON | OBJ_ONEMAPPING)) ==
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(OBJ_ANON | OBJ_ONEMAPPING)) {
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/*
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* Singly-mapped anonymous memory is discarded. This
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* does not match Linux's semantics when the object
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* belongs to a shadow chain of length > 1, since
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* subsequent faults may retrieve pages from an
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* intermediate anonymous object. However, handling
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* this case correctly introduces a fair bit of
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* complexity.
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*/
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VM_OBJECT_WLOCK(object);
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if ((object->flags & OBJ_ONEMAPPING) != 0) {
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vm_object_collapse(object);
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vm_object_page_remove(object, pstart, pend, 0);
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backing_object = object->backing_object;
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if (backing_object != NULL &&
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(backing_object->flags & OBJ_ANON) != 0)
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linux_msg(td,
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"possibly incorrect MADV_DONTNEED");
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VM_OBJECT_WUNLOCK(object);
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continue;
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}
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VM_OBJECT_WUNLOCK(object);
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}
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/*
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* Handle shared mappings. Remove them outright instead of
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* calling pmap_advise(), for consistency with Linux.
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*/
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pmap_remove(map->pmap, estart, eend);
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vm_object_madvise(object, pstart, pend, MADV_DONTNEED);
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}
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vm_map_unlock_read(map);
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return (error);
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}
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int
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int
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linux_madvise_common(struct thread *td, uintptr_t addr, size_t len, int behav)
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linux_madvise_common(struct thread *td, uintptr_t addr, size_t len, int behav)
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{
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{
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@ -256,7 +351,7 @@ linux_madvise_common(struct thread *td, uintptr_t addr, size_t len, int behav)
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case LINUX_MADV_WILLNEED:
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case LINUX_MADV_WILLNEED:
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return (kern_madvise(td, addr, len, MADV_WILLNEED));
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return (kern_madvise(td, addr, len, MADV_WILLNEED));
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case LINUX_MADV_DONTNEED:
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case LINUX_MADV_DONTNEED:
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return (kern_madvise(td, addr, len, MADV_DONTNEED));
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return (linux_madvise_dontneed(td, addr, addr + len));
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case LINUX_MADV_FREE:
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case LINUX_MADV_FREE:
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return (kern_madvise(td, addr, len, MADV_FREE));
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return (kern_madvise(td, addr, len, MADV_FREE));
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case LINUX_MADV_REMOVE:
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case LINUX_MADV_REMOVE:
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