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Handle the race between fork/vm_object_split() and faults.

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If fault started before vmspace_fork() locked the map, and then during
fork, vm_map_copy_entry()->vm_object_split() is executed, it is
possible that the fault instantiate the page into the original object
when the page was already copied into the new object (see
vm_map_split() for the orig/new objects terminology). This can happen
if split found a busy page (e.g. from the fault) and slept dropping
the objects lock, which allows the swap pager to instantiate
read-behind pages for the fault.  Then the restart of the scan can see
a page in the scanned range, where it was already copied to the upper
object.

Fix it by instantiating the read-ahead pages before
swap_pager_getpages() method drops the lock to allocate pbuf.  The
object scan would see the whole range prefilled with the busy pages
and not proceed the range.

Note that vm_fault rechecks the map generation count after the object
unlock, so that it restarts the handling if raced with split, and
re-lookups the right page from the upper object.

In collaboration with:	alc
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
This commit is contained in:
Konstantin Belousov 2018-06-14 19:41:02 +00:00
parent 7de871aa81
commit b7b8a09658
1 changed files with 30 additions and 28 deletions
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58
sys/vm/swap_pager.c
View File

@ -1096,21 +1096,24 @@ swap_pager_getpages(vm_object_t object, vm_page_t *ma, int count, int *rbehind,
int *rahead)
{
struct buf *bp;
vm_page_t mpred, msucc, p;
vm_page_t bm, mpred, msucc, p;
vm_pindex_t pindex;
daddr_t blk;
int i, j, maxahead, maxbehind, reqcount, shift;
int i, maxahead, maxbehind, reqcount;
reqcount = count;
VM_OBJECT_WUNLOCK(object);
bp = getpbuf(&nsw_rcount);
VM_OBJECT_WLOCK(object);
if (!swap_pager_haspage(object, ma[0]->pindex, &maxbehind, &maxahead)) {
relpbuf(bp, &nsw_rcount);
/*
* Determine the final number of read-behind pages and
* allocate them BEFORE releasing the object lock. Otherwise,
* there can be a problematic race with vm_object_split().
* Specifically, vm_object_split() might first transfer pages
* that precede ma[0] in the current object to a new object,
* and then this function incorrectly recreates those pages as
* read-behind pages in the current object.
*/
if (!swap_pager_haspage(object, ma[0]->pindex, &maxbehind, &maxahead))
return (VM_PAGER_FAIL);
}
/*
* Clip the readahead and readbehind ranges to exclude resident pages.
@ -1132,35 +1135,31 @@ swap_pager_getpages(vm_object_t object, vm_page_t *ma, int count, int *rbehind,
*rbehind = pindex - mpred->pindex - 1;
}
bm = ma[0];
for (i = 0; i < count; i++)
ma[i]->oflags |= VPO_SWAPINPROG;
/*
* Allocate readahead and readbehind pages.
*/
shift = rbehind != NULL ? *rbehind : 0;
if (shift != 0) {
for (i = 1; i <= shift; i++) {
if (rbehind != NULL) {
for (i = 1; i <= *rbehind; i++) {
p = vm_page_alloc(object, ma[0]->pindex - i,
VM_ALLOC_NORMAL);
if (p == NULL) {
/* Shift allocated pages to the left. */
for (j = 0; j < i - 1; j++)
bp->b_pages[j] =
bp->b_pages[j + shift - i + 1];
if (p == NULL)
break;
p->oflags |= VPO_SWAPINPROG;
bm = p;
}
bp->b_pages[shift - i] = p;
*rbehind = i - 1;
}
shift = i - 1;
*rbehind = shift;
}
for (i = 0; i < reqcount; i++)
bp->b_pages[i + shift] = ma[i];
if (rahead != NULL) {
for (i = 0; i < *rahead; i++) {
p = vm_page_alloc(object,
ma[reqcount - 1]->pindex + i + 1, VM_ALLOC_NORMAL);
if (p == NULL)
break;
bp->b_pages[shift + reqcount + i] = p;
p->oflags |= VPO_SWAPINPROG;
}
*rahead = i;
}
@ -1171,15 +1170,18 @@ swap_pager_getpages(vm_object_t object, vm_page_t *ma, int count, int *rbehind,
vm_object_pip_add(object, count);
for (i = 0; i < count; i++)
bp->b_pages[i]->oflags |= VPO_SWAPINPROG;
pindex = bp->b_pages[0]->pindex;
pindex = bm->pindex;
blk = swp_pager_meta_ctl(object, pindex, 0);
KASSERT(blk != SWAPBLK_NONE,
("no swap blocking containing %p(%jx)", object, (uintmax_t)pindex));
VM_OBJECT_WUNLOCK(object);
bp = getpbuf(&nsw_rcount);
/* Pages cannot leave the object while busy. */
for (i = 0, p = bm; i < count; i++, p = TAILQ_NEXT(p, listq)) {
MPASS(p->pindex == bm->pindex + i);
bp->b_pages[i] = p;
}
bp->b_flags |= B_PAGING;
bp->b_iocmd = BIO_READ;