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madvise(MADV_FREE): Quick fix to time rewind.

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Don't free pages in a shadowing object.  While this degrades MADV_FREE
to a no-op (and we could, instead, choose to fall back to
MADV_DONTNEED, at the cost of changing pmap_madvise), this is
presently considered a temporary fix. We may prefer to risk a little
fragmentation of the map by creating a zero/OBJT_DEFAULT entry over
top of the existing object and, simultaneously, revert to the existing
marking any pages in the former shadowing object in the advised region
as reclaimable.  At least one consumer of MADV_FREE (snmalloc) may use
mmap() to construct zeroed pages "eventually" here anyway, so the
fragmentation may be coming anyway.

Submitted by:	Nathaniel Filardo <nwf20@cl.cam.ac.uk>
PR:	240061
Reviewed by:	markj
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D21517
This commit is contained in:
kib 2019-09-04 20:28:16 +00:00
parent 362659d41a
commit 9c68b962c1
1 changed files with 12 additions and 0 deletions
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12
sys/vm/vm_map.c
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@ -2736,6 +2736,18 @@ vm_map_madvise(
if (current->eflags & MAP_ENTRY_IS_SUB_MAP)
continue;
/*
* MADV_FREE would otherwise rewind time to
* the creation of the shadow object. Because
* we hold the VM map read-locked, neither the
* entry's object nor the presence of a
* backing object can change.
*/
if (behav == MADV_FREE &&
current->object.vm_object != NULL &&
current->object.vm_object->backing_object != NULL)
continue;
pstart = OFF_TO_IDX(current->offset);
pend = pstart + atop(current->end - current->start);
useStart = current->start;