git: e123264e4dc3 - main - vm: Fix racy checks for swap objects
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Date: Mon, 20 Jun 2022 17:00:33 UTC
The branch main has been updated by markj: URL: https://cgit.FreeBSD.org/src/commit/?id=e123264e4dc394602f9fed2f0376204b5998d815 commit e123264e4dc394602f9fed2f0376204b5998d815 Author: Mark Johnston <markj@FreeBSD.org> AuthorDate: 2022-06-20 16:18:15 +0000 Commit: Mark Johnston <markj@FreeBSD.org> CommitDate: 2022-06-20 16:48:14 +0000 vm: Fix racy checks for swap objects Commit 4b8365d752ef introduced the ability to dynamically register VM object types, for use by tmpfs, which creates swap-backed objects. As a part of this, checks for such objects changed from object->type == OBJT_DEFAULT || object->type == OBJT_SWAP to object->type == OBJT_DEFAULT || (object->flags & OBJ_SWAP) != 0 In particular, objects of type OBJT_DEFAULT do not have OBJ_SWAP set; the swap pager sets this flag when converting from OBJT_DEFAULT to OBJT_SWAP. A few of these checks are done without the object lock held. It turns out that this can result in false negatives since the swap pager converts objects like so: object->type = OBJT_SWAP; object->flags |= OBJ_SWAP; Fix the problem by adding explicit tests for OBJT_SWAP objects in unlocked checks. PR: 258932 Fixes: 4b8365d752ef ("Add OBJT_SWAP_TMPFS pager") Reported by: bdrewery Reviewed by: kib MFC after: 2 weeks Sponsored by: The FreeBSD Foundation Differential Revision: https://reviews.freebsd.org/D35470 --- sys/vm/vm_map.c | 9 ++++++--- sys/vm/vm_mmap.c | 5 ++--- sys/vm/vm_pageout.c | 5 +++-- 3 files changed, 11 insertions(+), 8 deletions(-) diff --git a/sys/vm/vm_map.c b/sys/vm/vm_map.c index 1842b6e1a314..93182051abb7 100644 --- a/sys/vm/vm_map.c +++ b/sys/vm/vm_map.c @@ -2824,9 +2824,6 @@ again: continue; } - if (obj->type != OBJT_DEFAULT && - (obj->flags & OBJ_SWAP) == 0) - continue; VM_OBJECT_WLOCK(obj); if (obj->type != OBJT_DEFAULT && (obj->flags & OBJ_SWAP) == 0) { @@ -4139,7 +4136,13 @@ vm_map_copy_entry( */ size = src_entry->end - src_entry->start; if ((src_object = src_entry->object.vm_object) != NULL) { + /* + * Swap-backed objects need special handling. Note that + * this is an unlocked check, so it is possible to race + * with an OBJT_DEFAULT -> OBJT_SWAP conversion. + */ if (src_object->type == OBJT_DEFAULT || + src_object->type == OBJT_SWAP || (src_object->flags & OBJ_SWAP) != 0) { vm_map_copy_swap_object(src_entry, dst_entry, size, fork_charge); diff --git a/sys/vm/vm_mmap.c b/sys/vm/vm_mmap.c index db9a32d1c9bc..5f27b550b78d 100644 --- a/sys/vm/vm_mmap.c +++ b/sys/vm/vm_mmap.c @@ -1368,9 +1368,8 @@ vm_mmap_vnode(struct thread *td, vm_size_t objsize, goto done; } } else { - KASSERT(obj->type == OBJT_DEFAULT || - (obj->flags & OBJ_SWAP) != 0, - ("wrong object type")); + KASSERT(obj->type == OBJT_DEFAULT || obj->type == OBJT_SWAP || + (obj->flags & OBJ_SWAP) != 0, ("wrong object type")); vm_object_reference(obj); #if VM_NRESERVLEVEL > 0 if ((obj->flags & OBJ_COLORED) == 0) { diff --git a/sys/vm/vm_pageout.c b/sys/vm/vm_pageout.c index 36d5f3275800..74439d5884ef 100644 --- a/sys/vm/vm_pageout.c +++ b/sys/vm/vm_pageout.c @@ -1886,8 +1886,9 @@ vm_pageout_oom_pagecount(struct vmspace *vmspace) if ((entry->eflags & MAP_ENTRY_NEEDS_COPY) != 0 && obj->ref_count != 1) continue; - if (obj->type == OBJT_DEFAULT || obj->type == OBJT_PHYS || - obj->type == OBJT_VNODE || (obj->flags & OBJ_SWAP) != 0) + if (obj->type == OBJT_DEFAULT || obj->type == OBJT_SWAP || + obj->type == OBJT_PHYS || obj->type == OBJT_VNODE || + (obj->flags & OBJ_SWAP) != 0) res += obj->resident_page_count; } return (res);