On Mon, 5 Nov 2018 16:22:25 +0100 Vitaly Wool <vitalywool@xxxxxxxxx> wrote: > 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". This conflicts with z3fold-fix-wrong-handling-of-headless-pages.patch, below. What should we do? (I think we're still awaiting your input on this one. Or I might have missed an amail.) From: Jongseok Kim <ks77sj@xxxxxxxxx> Subject: mm/z3fold.c: fix wrong handling of headless pages During the processing of headless pages in z3fold_reclaim_page(), there was a problem that the zhdr pointed to another page or a page was already released in z3fold_free(). So, the wrong page is encoded in headless, or test_bit does not work properly in z3fold_reclaim_page(). This patch fixed these problems. Link: http://lkml.kernel.org/r/1530853846-30215-1-git-send-email-ks77sj@xxxxxxxxx Signed-off-by: Jongseok Kim <ks77sj@xxxxxxxxx> Cc: Vitaly Wool <vitalywool@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/z3fold.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) --- a/mm/z3fold.c~z3fold-fix-wrong-handling-of-headless-pages +++ a/mm/z3fold.c @@ -746,6 +746,9 @@ static void z3fold_free(struct z3fold_po } if (bud == HEADLESS) { + if (test_bit(UNDER_RECLAIM, &page->private)) + return; + spin_lock(&pool->lock); list_del(&page->lru); spin_unlock(&pool->lock); @@ -836,20 +839,20 @@ static int z3fold_reclaim_page(struct z3 } list_for_each_prev(pos, &pool->lru) { page = list_entry(pos, struct page, lru); + zhdr = page_address(page); if (test_bit(PAGE_HEADLESS, &page->private)) /* candidate found */ break; - zhdr = page_address(page); if (!z3fold_page_trylock(zhdr)) 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; } + set_bit(UNDER_RECLAIM, &page->private); list_del_init(&page->lru); spin_unlock(&pool->lock); @@ -898,6 +901,7 @@ next: 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); _
