On Thu, Aug 22, 2024 at 11:13 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > On Tue, Aug 20, 2024 at 11:42 PM Kairui Song <ryncsn@xxxxxxxxx> wrote: > > > > On Wed, Aug 21, 2024 at 1:49 PM Barry Song <21cnbao@xxxxxxxxx> wrote: > > > > > > On Tue, Aug 20, 2024 at 9:02 PM Kairui Song <ryncsn@xxxxxxxxx> wrote: > > > > > > > > On Tue, Aug 20, 2024 at 4:47 PM Kairui Song <ryncsn@xxxxxxxxx> wrote: > > > > > > > > > > On Tue, Aug 20, 2024 at 4:13 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > > > > > On Fri, Aug 16, 2024 at 12:52 PM syzbot > > > > > > <syzbot+ce6029250d7fd4d0476d@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > > > > > > > Hello, > > > > > > > > > > > > > > syzbot found the following issue on: > > > > > > > > > > > > > > HEAD commit: 367b5c3d53e5 Add linux-next specific files for 20240816 > > > > > > > > > > I can't find this commit, seems this commit is not in linux-next any more? > > > > > > > > > > > > git tree: linux-next > > > > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=12489105980000 > > > > > > > kernel config: https://syzkaller.appspot.com/x/.config?x=61ba6f3b22ee5467 > > > > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=ce6029250d7fd4d0476d > > > > > > > compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40 > > > > > > > > > > > > > > Unfortunately, I don't have any reproducer for this issue yet. > > > > > > > > > > > > > > Downloadable assets: > > > > > > > disk image: https://storage.googleapis.com/syzbot-assets/0b1b4e3cad3c/disk-367b5c3d.raw.xz > > > > > > > vmlinux: https://storage.googleapis.com/syzbot-assets/5bb090f7813c/vmlinux-367b5c3d.xz > > > > > > > kernel image: https://storage.googleapis.com/syzbot-assets/6674cb0709b1/bzImage-367b5c3d.xz > > > > > > > > > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > > > > > > > Reported-by: syzbot+ce6029250d7fd4d0476d@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > > > > > > > > > > > ------------[ cut here ]------------ > > > > > > > WARNING: CPU: 0 PID: 11298 at mm/zswap.c:1700 zswap_swapoff+0x11b/0x2b0 mm/zswap.c:1700 > > > > > > > Modules linked in: > > > > > > > CPU: 0 UID: 0 PID: 11298 Comm: swapoff Not tainted 6.11.0-rc3-next-20240816-syzkaller #0 > > > > > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/27/2024 > > > > > > > RIP: 0010:zswap_swapoff+0x11b/0x2b0 mm/zswap.c:1700 > > > > > > > Code: 74 05 e8 78 73 07 00 4b 83 7c 35 00 00 75 15 e8 1b bd 9e ff 48 ff c5 49 83 c6 50 83 7c 24 0c 17 76 9b eb 24 e8 06 bd 9e ff 90 <0f> 0b 90 eb e5 48 8b 0c 24 80 e1 07 80 c1 03 38 c1 7c 90 48 8b 3c > > > > > > > RSP: 0018:ffffc9000302fa38 EFLAGS: 00010293 > > > > > > > RAX: ffffffff81f4d66a RBX: dffffc0000000000 RCX: ffff88802c19bc00 > > > > > > > RDX: 0000000000000000 RSI: 0000000000000002 RDI: ffff888015986248 > > > > > > > RBP: 0000000000000000 R08: ffffffff81f4d620 R09: 1ffffffff1d476ac > > > > > > > R10: dffffc0000000000 R11: fffffbfff1d476ad R12: dffffc0000000000 > > > > > > > R13: ffff888015986200 R14: 0000000000000048 R15: 0000000000000002 > > > > > > > FS: 00007f9e628a5380(0000) GS:ffff8880b9000000(0000) knlGS:0000000000000000 > > > > > > > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > > > > > > > CR2: 0000001b30f15ff8 CR3: 000000006c5f0000 CR4: 00000000003506f0 > > > > > > > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 > > > > > > > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 > > > > > > > Call Trace: > > > > > > > <TASK> > > > > > > > __do_sys_swapoff mm/swapfile.c:2837 [inline] > > > > > > > __se_sys_swapoff+0x4653/0x4cf0 mm/swapfile.c:2706 > > > > > > > do_syscall_x64 arch/x86/entry/common.c:52 [inline] > > > > > > > do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83 > > > > > > > entry_SYSCALL_64_after_hwframe+0x77/0x7f > > > > > > > RIP: 0033:0x7f9e629feb37 > > > > > > > Code: 73 01 c3 48 8b 0d f1 52 0d 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 b8 a8 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d c1 52 0d 00 f7 d8 64 89 01 48 > > > > > > > RSP: 002b:00007fff17734f68 EFLAGS: 00000246 ORIG_RAX: 00000000000000a8 > > > > > > > RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f9e629feb37 > > > > > > > RDX: 00007f9e62a9e7e8 RSI: 00007f9e62b9beed RDI: 0000563090942a20 > > > > > > > RBP: 0000563090942a20 R08: 0000000000000000 R09: 77872e07ed164f94 > > > > > > > R10: 000000000000001f R11: 0000000000000246 R12: 00007fff17735188 > > > > > > > R13: 00005630909422a0 R14: 0000563073724169 R15: 00007f9e62bdda80 > > > > > > > </TASK> > > > > > > > > > > > > I am hoping syzbot would find a reproducer and bisect this for us. > > > > > > Meanwhile, from a high-level it looks to me like we are missing a > > > > > > zswap_invalidate() call in some paths. > > > > > > > > > > > > If I have to guess, I would say it's related to the latest mTHP swap > > > > > > changes, but I am not following closely. Perhaps one of the following > > > > > > things happened: > > > > > > > > > > > > (1) We are not calling zswap_invalidate() in some invalidation paths. > > > > > > It used to not be called for the cluster freeing path, so maybe we end > > > > > > up with some order-0 swap entries in a cluster? or maybe there is an > > > > > > entirely new invalidation path that does not go through > > > > > > free_swap_slot() for order-0 entries? > > > > > > > > > > > > (2) Some higher order swap entries (i.e. a cluster) end up in zswap > > > > > > somehow. zswap_store() has a warning to cover that though. Maybe > > > > > > somehow some swap entries are allocated as a cluster, but then pages > > > > > > are swapped out one-by-one as order-0 (which can go to zswap), but > > > > > > then we still free the swap entries as a cluster? > > > > > > > > > > Hi Yosry, thanks for the report. > > > > > > > > > > There are many mTHP related optimizations recently, for this problem I > > > > > can reproduce this locally. Can confirm the problem is gone for me > > > > > after reverting: > > > > > > > > > > "mm: attempt to batch free swap entries for zap_pte_range()" > > > > > > > > > > Hi Barry, > > > > > > > > > > If a set of continuous slots are having the same value, they are > > > > > considered a mTHP and freed, bypassing the slot cache, and causing > > > > > zswap leak. > > > > > This didn't happen in put_swap_folio because that function is > > > > > expecting an actual mTHP folio behind the slots but > > > > > free_swap_and_cache_nr is simply walking the slots. > > > > > > > > > > For the testing, I actually have to disable mTHP, because linux-next > > > > > will panic with mTHP due to lack of following fixes: > > > > > https://lore.kernel.org/linux-mm/a4b1b34f-0d8c-490d-ab00-eaedbf3fe780@xxxxxxxxx/ > > > > > https://lore.kernel.org/linux-mm/403b7f3c-6e5b-4030-ab1c-3198f36e3f73@xxxxxxxxx/ > > > > > > > > > > > > > > > > > I am not closely following the latest changes so I am not sure. CCing > > > > > > folks who have done work in that area recently. > > > > > > > > > > > > I am starting to think maybe it would be more reliable to just call > > > > > > zswap_invalidate() for all freed swap entries anyway. Would that be > > > > > > too expensive? We used to do that before the zswap_invalidate() call > > > > > > was moved by commit 0827a1fb143f ("mm/zswap: invalidate zswap entry > > > > > > when swap entry free"), and that was before we started using the > > > > > > xarray (so it was arguably worse than it would be now). > > > > > > > > > > > > > > > > That might be a good idea, I suggest moving zswap_invalidate to > > > > > swap_range_free and call it for every freed slot. > > > > > > > > > > Below patch can be squash into or put before "mm: attempt to batch > > > > > free swap entries for zap_pte_range()". > > > > > > > > Hmm, on second thought, the commit message in the attachment commit > > > > might be not suitable, current zswap_invalidate is also designed to > > > > only work for order 0 ZSWAP, so things are not clean even after this. > > > > > > Kairui, what about the below? we don't touch the path of __try_to_reclaim_swap() where > > > you have one folio backed? > > > > > > diff --git a/mm/swapfile.c b/mm/swapfile.c > > > index c1638a009113..8ff58be40544 100644 > > > --- a/mm/swapfile.c > > > +++ b/mm/swapfile.c > > > @@ -1514,6 +1514,8 @@ static bool __swap_entries_free(struct swap_info_struct *si, > > > unlock_cluster_or_swap_info(si, ci); > > > > > > if (!has_cache) { > > > + for (i = 0; i < nr; i++) > > > + zswap_invalidate(swp_entry(si->type, offset + i)); > > > spin_lock(&si->lock); > > > swap_entry_range_free(si, entry, nr); > > > spin_unlock(&si->lock); > > > > > > > Hi Barry, > > > > Thanks for updating this thread, I'm thinking maybe something will > > better be done at the zswap side? > > > > The concern of using zswap_invalidate is that it calls xa_erase which > > requires the xa spin lock. But if we are calling zswap_invalidate in > > swap_entry_range_free, and ensure the slot is HAS_CACHE pinned, doing > > a lockless read first with xa_load should be OK for checking if the > > slot needs a ZSWAP invalidation. The performance cost will be minimal > > and we only need to call zswap_invalidate in one place, something like > > this (haven't tested, comments are welcome). Also ZSWAP mthp will > > still store entried in order 0 so this should be OK for future. > > > While I do agree with this change on a high level, it's essentially > reverting commit 0827a1fb143f ("mm/zswap: invalidate zswap entry when > swap entry free") which fixed a small problem with zswap writeback. > I'd prefer that we don't if possible. > > One thing that I always wanted to do is to pull some of the work done > in swap_entry_range_free() and swap_range_free() before the slots > caching layer. The memcg uncharging, clearing shadow entries from the > swap cache, arch invalidation, zswap invalidation, etc. If we can have > a hook for these pre-free callbacks we can call it for single entries > before we add them to the slots cache, and call them for the clusters > as we do today. This should also reduce the amount of work done under > the lock, and move more work to where the freeing is actually > happening vs. the cache draining. > > I remember discussing this briefly with Ying before. Anyone have any thoughts? Hi Yosry, If I understand correctly, the lock you are talking about is the si->lock, right? Kairui has some WIP patches removing the swap slot cache in the swap entry freeing path. Basically the si->lock is only used to protect the cluster list. Most of the time freeing swap entry will only take the ci->lock. No need to take the si->lock to change the cluster lists. Only when the cluster moves to another list will it require the si->lock e.g. the cluster moves to the free list when all 512 entries are freed. Because each cluster has 512 entries. The need to take si->lock is dramatically reduced. That patch is based on the new cluster swap allocator series. Kairui can share more details. I don't think ci->lock has heavy contentions. Chris > > > > > diff --git a/mm/swap_slots.c b/mm/swap_slots.c > > index 13ab3b771409..d7bb3caa9d4e 100644 > > --- a/mm/swap_slots.c > > +++ b/mm/swap_slots.c > > @@ -273,9 +273,6 @@ void free_swap_slot(swp_entry_t entry) > > { > > struct swap_slots_cache *cache; > > > > - /* Large folio swap slot is not covered. */ > > - zswap_invalidate(entry); > > - > > cache = raw_cpu_ptr(&swp_slots); > > if (likely(use_swap_slot_cache && cache->slots_ret)) { > > spin_lock_irq(&cache->free_lock); > > diff --git a/mm/swapfile.c b/mm/swapfile.c > > index f947f4dd31a9..fbc25d38a27e 100644 > > --- a/mm/swapfile.c > > +++ b/mm/swapfile.c > > @@ -242,9 +242,6 @@ static int __try_to_reclaim_swap(struct > > swap_info_struct *si, > > folio_set_dirty(folio); > > > > spin_lock(&si->lock); > > - /* Only sinple page folio can be backed by zswap */ > > - if (nr_pages == 1) > > - zswap_invalidate(entry); > > swap_entry_range_free(si, entry, nr_pages); > > spin_unlock(&si->lock); > > ret = nr_pages; > > @@ -1545,6 +1542,10 @@ static void swap_entry_range_free(struct > > swap_info_struct *si, swp_entry_t entry > > unsigned char *map_end = map + nr_pages; > > struct swap_cluster_info *ci; > > > > + /* Slots are pinned with SWAP_HAS_CACHE, safe to invalidate */ > > + for (int i = 0; i < nr_pages; ++i) > > + zswap_invalidate(swp_entry(si->type, offset + i)); > > + > > ci = lock_cluster(si, offset); > > do { > > VM_BUG_ON(*map != SWAP_HAS_CACHE); > > diff --git a/mm/zswap.c b/mm/zswap.c > > index df66ab102d27..100ad04397fe 100644 > > --- a/mm/zswap.c > > +++ b/mm/zswap.c > > @@ -1656,15 +1656,18 @@ bool zswap_load(struct folio *folio) > > return true; > > } > > > > +/* Caller need to pin the slot to prevent parallel store */ > > void zswap_invalidate(swp_entry_t swp) > > { > > pgoff_t offset = swp_offset(swp); > > struct xarray *tree = swap_zswap_tree(swp); > > struct zswap_entry *entry; > > > > - entry = xa_erase(tree, offset); > > - if (entry) > > - zswap_entry_free(entry); > > + if (xa_load(tree, offset)) { > > + entry = xa_erase(tree, offset); > > + if (entry) > > + zswap_entry_free(entry); > > + } > > } > > > > int zswap_swapon(int type, unsigned long nr_pages) > > -- > > 2.45.2
