On Tue 05-04-22 12:20:15, Alexander Sverdlin wrote: > Dear mm developers! > > After experiencing a crash in release_pages() [1] I'm trying to understand the locking in the release_pages(): > > No matter if we consider v5.17 or v5.4 (as in my case), they both have similar locking patterns: Similar but the notable difference is that 5.4 used per node lru locking while newer versions 5.11+ kernels use per memcg locking. If you see the issue on 5.4 then this is unlikely a regression. [...] > What I don't understand here is, what guarantees us that "if (PageLRU(page))" condition > is still valid after we swap the locks in "if (pgdat != locked_pgdat)" case? The underlying reasoning is that the PageLRU handling is done after the last reference has been dropped. isolate_lru_page and others should elevate the reference count before isolating page from LRU lists. Some callers user TestClearPageLRU > If we check under one lock and VM_BUG_ON_PAGE() under another lock, what actually stops > it from crashing as below or BUG() from time to time? G > > 1. Crash of v5.4.170 on an ARM32 machine: > > Unable to handle kernel NULL pointer dereference at virtual address 00000104 > pgd = e138149d > [00000104] *pgd=84d2fd003, *pmd=8ffd6f003 > Internal error: Oops: a07 [#1] PREEMPT SMP ARM > ... > CPU: 1 PID: 6172 Comm: AaSysInfoRColle Tainted: G B O 5.4.170-... #1 > Hardware name: Keystone > PC is at release_pages+0x194/0x358 > LR is at release_pages+0x10c/0x358 Which LOC does this correspond to? (faddr2line should give you a nice output). -- Michal Hocko SUSE Labs
