Messages in this thread | | | Subject | Re: [PATCH] mm/filemap: Adding missing mem_cgroup_uncharge() to __add_to_page_cache_locked() | From | Waiman Long <> | Date | Mon, 25 Jan 2021 13:57:18 -0500 |
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On 1/25/21 1:52 PM, Johannes Weiner wrote: > On Mon, Jan 25, 2021 at 01:23:58PM -0500, Waiman Long wrote: >> On 1/25/21 1:14 PM, Michal Hocko wrote: >>> On Mon 25-01-21 17:41:19, Michal Hocko wrote: >>>> On Mon 25-01-21 16:25:06, Matthew Wilcox wrote: >>>>> On Mon, Jan 25, 2021 at 05:03:28PM +0100, Michal Hocko wrote: >>>>>> On Mon 25-01-21 10:57:54, Waiman Long wrote: >>>>>>> On 1/25/21 4:28 AM, Michal Hocko wrote: >>>>>>>> On Sun 24-01-21 23:24:41, Waiman Long wrote: >>>>>>>>> The commit 3fea5a499d57 ("mm: memcontrol: convert page >>>>>>>>> cache to a new mem_cgroup_charge() API") introduced a bug in >>>>>>>>> __add_to_page_cache_locked() causing the following splat: >>>>>>>>> >>>>>>>>> [ 1570.068330] page dumped because: VM_BUG_ON_PAGE(page_memcg(page)) >>>>>>>>> [ 1570.068333] pages's memcg:ffff8889a4116000 >>>>>>>>> [ 1570.068343] ------------[ cut here ]------------ >>>>>>>>> [ 1570.068346] kernel BUG at mm/memcontrol.c:2924! >>>>>>>>> [ 1570.068355] invalid opcode: 0000 [#1] SMP KASAN PTI >>>>>>>>> [ 1570.068359] CPU: 35 PID: 12345 Comm: cat Tainted: G S W I 5.11.0-rc4-debug+ #1 >>>>>>>>> [ 1570.068363] Hardware name: HP HP Z8 G4 Workstation/81C7, BIOS P60 v01.25 12/06/2017 >>>>>>>>> [ 1570.068365] RIP: 0010:commit_charge+0xf4/0x130 >>>>>>>>> : >>>>>>>>> [ 1570.068375] RSP: 0018:ffff8881b38d70e8 EFLAGS: 00010286 >>>>>>>>> [ 1570.068379] RAX: 0000000000000000 RBX: ffffea00260ddd00 RCX: 0000000000000027 >>>>>>>>> [ 1570.068382] RDX: 0000000000000000 RSI: 0000000000000004 RDI: ffff88907ebe05a8 >>>>>>>>> [ 1570.068384] RBP: ffffea00260ddd00 R08: ffffed120fd7c0b6 R09: ffffed120fd7c0b6 >>>>>>>>> [ 1570.068386] R10: ffff88907ebe05ab R11: ffffed120fd7c0b5 R12: ffffea00260ddd38 >>>>>>>>> [ 1570.068389] R13: ffff8889a4116000 R14: ffff8889a4116000 R15: 0000000000000001 >>>>>>>>> [ 1570.068391] FS: 00007ff039638680(0000) GS:ffff88907ea00000(0000) knlGS:0000000000000000 >>>>>>>>> [ 1570.068394] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 >>>>>>>>> [ 1570.068396] CR2: 00007f36f354cc20 CR3: 00000008a0126006 CR4: 00000000007706e0 >>>>>>>>> [ 1570.068398] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 >>>>>>>>> [ 1570.068400] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 >>>>>>>>> [ 1570.068402] PKRU: 55555554 >>>>>>>>> [ 1570.068404] Call Trace: >>>>>>>>> [ 1570.068407] mem_cgroup_charge+0x175/0x770 >>>>>>>>> [ 1570.068413] __add_to_page_cache_locked+0x712/0xad0 >>>>>>>>> [ 1570.068439] add_to_page_cache_lru+0xc5/0x1f0 >>>>>>>>> [ 1570.068461] cachefiles_read_or_alloc_pages+0x895/0x2e10 [cachefiles] >>>>>>>>> [ 1570.068524] __fscache_read_or_alloc_pages+0x6c0/0xa00 [fscache] >>>>>>>>> [ 1570.068540] __nfs_readpages_from_fscache+0x16d/0x630 [nfs] >>>>>>>>> [ 1570.068585] nfs_readpages+0x24e/0x540 [nfs] >>>>>>>>> [ 1570.068693] read_pages+0x5b1/0xc40 >>>>>>>>> [ 1570.068711] page_cache_ra_unbounded+0x460/0x750 >>>>>>>>> [ 1570.068729] generic_file_buffered_read_get_pages+0x290/0x1710 >>>>>>>>> [ 1570.068756] generic_file_buffered_read+0x2a9/0xc30 >>>>>>>>> [ 1570.068832] nfs_file_read+0x13f/0x230 [nfs] >>>>>>>>> [ 1570.068872] new_sync_read+0x3af/0x610 >>>>>>>>> [ 1570.068901] vfs_read+0x339/0x4b0 >>>>>>>>> [ 1570.068909] ksys_read+0xf1/0x1c0 >>>>>>>>> [ 1570.068920] do_syscall_64+0x33/0x40 >>>>>>>>> [ 1570.068926] entry_SYSCALL_64_after_hwframe+0x44/0xa9 >>>>>>>>> [ 1570.068930] RIP: 0033:0x7ff039135595 >>>>>>>>> >>>>>>>>> Before that commit, there was a try_charge() and commit_charge() >>>>>>>>> in __add_to_page_cache_locked(). These 2 separated charge functions >>>>>>>>> were replaced by a single mem_cgroup_charge(). However, it forgot >>>>>>>>> to add a matching mem_cgroup_uncharge() when the xarray insertion >>>>>>>>> failed with the page released back to the pool. Fix this by adding a >>>>>>>>> mem_cgroup_uncharge() call when insertion error happens. >>>>>>>>> >>>>>>>>> Fixes: 3fea5a499d57 ("mm: memcontrol: convert page cache to a new mem_cgroup_charge() API") >>>>>>>>> Signed-off-by: Waiman Long <longman@redhat.com> >>>>>>>> OK, this is indeed a subtle bug. The patch aimed at simplifying the >>>>>>>> charge lifetime so that users do not really have to think about when to >>>>>>>> uncharge as that happens when the page is freed. fscache somehow breaks >>>>>>>> that assumption because it doesn't free up pages but it keeps some of >>>>>>>> them in the cache. >>>>>>>> >>>>>>>> I have tried to wrap my head around the cached object life time in >>>>>>>> fscache but failed and got lost in the maze. Is this the only instance >>>>>>>> of the problem? Would it make more sense to explicitly handle charges in >>>>>>>> the fscache code or there are other potential users to fall into this >>>>>>>> trap? >>>>>>> There may be other places that have similar problem. I focus on the >>>>>>> filemap.c case as I have a test case that can reliably produce the bug >>>>>>> splat. This patch does fix it for my test case. >>>>>> I believe this needs a more general fix than catching a random places >>>>>> which you can trigger. Would it make more sense to address this at the >>>>>> fscache level and always make sure that a page returned to the pool is >>>>>> always uncharged instead? >>>>> I believe you mean "page cache" -- there is a separate thing called >>>>> 'fscache' which is used to cache network filesystems. >>>> Yes, I really had fscache in mind because it does have an "unusual" page >>>> life time rules. >>>> >>>>> I don't understand the memcg code at all, so I have no useful feedback >>>>> on what you're saying other than this. >>>> Well the memcg accounting rules after the rework should have simplified >>>> the API usage for most users. You will get memory charged when it is >>>> used and it will go away when the page is freed. If a page is not really >>>> freed in some cases and it can be reused then it doesn't really fit into >>>> this scheme automagically. I do undestand that this puts some additional >>>> burden on those special cases. I am not really sure what is the right >>>> way here myself but considering there might be other similar cases like >>>> that I would lean towards special casing where the pool is implemented. >>>> I would expect there is some state to be maintain for that purpose >>>> already. >>> After some more thinking I've came to conclusion that the patch as >>> proposed is the proper way forward. It is easier to follow if the >>> unwinding of state changes are local to the function. >> I think so. It is easier to understand if the charge and uncharge functions >> are grouped together in the same function. >>> With the proposed simplification by Willy >>> Acked-by: Michal Hocko <mhocko@suse.com> >> Thank for the ack. However, I am a bit confused about what you mean by >> simplification. There is another linux-next patch that changes the condition >> for mem_cgroup_charge() to >> >> - if (!huge) { >> + if (!huge && !page_is_secretmem(page)) { >> error = mem_cgroup_charge(page, current->mm, gfp); >> >> That is the main reason why I introduced the boolean variable as I don't >> want to call the external page_is_secretmem() function twice. > The variable works for me. > > On the other hand, as Michal points out, the uncharge function will be > called again on the page when it's being freed (in non-fscache cases), > so you're already relying on being able to call it on any page - > charged, uncharged, never charged. It would be fine to call it > unconditionally in the error path. Aesthetic preference, I guess.
That may be true. However, I haven't fully studied how the huge page memory accounting work to make sure the uncharge function can be called for huge pages. So I will keep the current code for now.
Thanks, Longman
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