z3fold: fix possible reclaim races
[ Upstream commit ca0246bb97c23da9d267c2107c07fb77e38205c9 ]
Reclaim and free can race on an object which is basically fine but in
order for reclaim to be able to map "freed" object we need to encode
object length in the handle. handle_to_chunks() is then introduced to
extract object length from a handle and use it during mapping.
Moreover, to avoid racing on a z3fold "headless" page release, we should
not try to free that page in z3fold_free() if the reclaim bit is set.
Also, in the unlikely case of trying to reclaim a page being freed, we
should not proceed with that page.
While at it, fix the page accounting in reclaim function.
This patch supersedes "[PATCH] z3fold: fix reclaim lock-ups".
Link: http://lkml.kernel.org/r/20181105162225.74e8837d03583a9b707cf559@gmail.com
Signed-off-by: Vitaly Wool <vitaly.vul@sony.com>
Signed-off-by: Jongseok Kim <ks77sj@gmail.com>
Reported-by-by: Jongseok Kim <ks77sj@gmail.com>
Reviewed-by: Snild Dolkow <snild@sony.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
diff --git a/mm/z3fold.c b/mm/z3fold.c
index f33403d..2813cdf 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -99,6 +99,7 @@
#define NCHUNKS ((PAGE_SIZE - ZHDR_SIZE_ALIGNED) >> CHUNK_SHIFT)
#define BUDDY_MASK (0x3)
+#define BUDDY_SHIFT 2
/**
* struct z3fold_pool - stores metadata for each z3fold pool
@@ -145,7 +146,7 @@
MIDDLE_CHUNK_MAPPED,
NEEDS_COMPACTING,
PAGE_STALE,
- UNDER_RECLAIM
+ PAGE_CLAIMED, /* by either reclaim or free */
};
/*****************
@@ -174,7 +175,7 @@
clear_bit(MIDDLE_CHUNK_MAPPED, &page->private);
clear_bit(NEEDS_COMPACTING, &page->private);
clear_bit(PAGE_STALE, &page->private);
- clear_bit(UNDER_RECLAIM, &page->private);
+ clear_bit(PAGE_CLAIMED, &page->private);
spin_lock_init(&zhdr->page_lock);
kref_init(&zhdr->refcount);
@@ -223,8 +224,11 @@
unsigned long handle;
handle = (unsigned long)zhdr;
- if (bud != HEADLESS)
- handle += (bud + zhdr->first_num) & BUDDY_MASK;
+ if (bud != HEADLESS) {
+ handle |= (bud + zhdr->first_num) & BUDDY_MASK;
+ if (bud == LAST)
+ handle |= (zhdr->last_chunks << BUDDY_SHIFT);
+ }
return handle;
}
@@ -234,6 +238,12 @@
return (struct z3fold_header *)(handle & PAGE_MASK);
}
+/* only for LAST bud, returns zero otherwise */
+static unsigned short handle_to_chunks(unsigned long handle)
+{
+ return (handle & ~PAGE_MASK) >> BUDDY_SHIFT;
+}
+
/*
* (handle & BUDDY_MASK) < zhdr->first_num is possible in encode_handle
* but that doesn't matter. because the masking will result in the
@@ -717,37 +727,39 @@
page = virt_to_page(zhdr);
if (test_bit(PAGE_HEADLESS, &page->private)) {
- /* HEADLESS page stored */
- bud = HEADLESS;
- } else {
- z3fold_page_lock(zhdr);
- bud = handle_to_buddy(handle);
-
- switch (bud) {
- case FIRST:
- zhdr->first_chunks = 0;
- break;
- case MIDDLE:
- zhdr->middle_chunks = 0;
- zhdr->start_middle = 0;
- break;
- case LAST:
- zhdr->last_chunks = 0;
- break;
- default:
- pr_err("%s: unknown bud %d\n", __func__, bud);
- WARN_ON(1);
- z3fold_page_unlock(zhdr);
- return;
+ /* if a headless page is under reclaim, just leave.
+ * NB: we use test_and_set_bit for a reason: if the bit
+ * has not been set before, we release this page
+ * immediately so we don't care about its value any more.
+ */
+ if (!test_and_set_bit(PAGE_CLAIMED, &page->private)) {
+ spin_lock(&pool->lock);
+ list_del(&page->lru);
+ spin_unlock(&pool->lock);
+ free_z3fold_page(page);
+ atomic64_dec(&pool->pages_nr);
}
+ return;
}
- if (bud == HEADLESS) {
- spin_lock(&pool->lock);
- list_del(&page->lru);
- spin_unlock(&pool->lock);
- free_z3fold_page(page);
- atomic64_dec(&pool->pages_nr);
+ /* Non-headless case */
+ z3fold_page_lock(zhdr);
+ bud = handle_to_buddy(handle);
+
+ switch (bud) {
+ case FIRST:
+ zhdr->first_chunks = 0;
+ break;
+ case MIDDLE:
+ zhdr->middle_chunks = 0;
+ break;
+ case LAST:
+ zhdr->last_chunks = 0;
+ break;
+ default:
+ pr_err("%s: unknown bud %d\n", __func__, bud);
+ WARN_ON(1);
+ z3fold_page_unlock(zhdr);
return;
}
@@ -755,7 +767,7 @@
atomic64_dec(&pool->pages_nr);
return;
}
- if (test_bit(UNDER_RECLAIM, &page->private)) {
+ if (test_bit(PAGE_CLAIMED, &page->private)) {
z3fold_page_unlock(zhdr);
return;
}
@@ -833,20 +845,30 @@
}
list_for_each_prev(pos, &pool->lru) {
page = list_entry(pos, struct page, lru);
- if (test_bit(PAGE_HEADLESS, &page->private))
- /* candidate found */
- break;
+
+ /* this bit could have been set by free, in which case
+ * we pass over to the next page in the pool.
+ */
+ if (test_and_set_bit(PAGE_CLAIMED, &page->private))
+ continue;
zhdr = page_address(page);
- if (!z3fold_page_trylock(zhdr))
+ if (test_bit(PAGE_HEADLESS, &page->private))
+ break;
+
+ if (!z3fold_page_trylock(zhdr)) {
+ zhdr = NULL;
continue; /* can't evict at this point */
+ }
kref_get(&zhdr->refcount);
list_del_init(&zhdr->buddy);
zhdr->cpu = -1;
- set_bit(UNDER_RECLAIM, &page->private);
break;
}
+ if (!zhdr)
+ break;
+
list_del_init(&page->lru);
spin_unlock(&pool->lock);
@@ -895,6 +917,7 @@
if (test_bit(PAGE_HEADLESS, &page->private)) {
if (ret == 0) {
free_z3fold_page(page);
+ atomic64_dec(&pool->pages_nr);
return 0;
}
spin_lock(&pool->lock);
@@ -902,7 +925,7 @@
spin_unlock(&pool->lock);
} else {
z3fold_page_lock(zhdr);
- clear_bit(UNDER_RECLAIM, &page->private);
+ clear_bit(PAGE_CLAIMED, &page->private);
if (kref_put(&zhdr->refcount,
release_z3fold_page_locked)) {
atomic64_dec(&pool->pages_nr);
@@ -961,7 +984,7 @@
set_bit(MIDDLE_CHUNK_MAPPED, &page->private);
break;
case LAST:
- addr += PAGE_SIZE - (zhdr->last_chunks << CHUNK_SHIFT);
+ addr += PAGE_SIZE - (handle_to_chunks(handle) << CHUNK_SHIFT);
break;
default:
pr_err("unknown buddy id %d\n", buddy);