Yang Shi <shy828301@xxxxxxxxx> writes: > On Tue, Sep 27, 2022 at 1:35 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote: >> >> On 9/26/22 18:51, Huang, Ying wrote: >> >>> But there might be other cases which may incur deadlock, for example, >> >>> filesystem writeback IIUC. Some filesystems may lock a bunch of pages >> >>> then write them back in a batch. The same pages may be on the >> >>> migration list and they are also dirty and seen by writeback. I'm not >> >>> sure whether I miss something that could prevent such a deadlock from >> >>> happening. >> >> >> >> I'm not overly familiar with that area but I would assume any filesystem >> >> code doing this would already have to deal with deadlock potential. >> > >> > Thank you very much for pointing this out. I think the deadlock is a >> > real issue. Anyway, we shouldn't forbid other places in kernel to lock >> > 2 pages at the same time. >> > >> >> I also agree that we cannot make any rules such as "do not lock > 1 page >> at the same time, elsewhere in the kernel", because it is already >> happening, for example in page-writeback.c, which locks PAGEVEC_SIZE >> (15) pages per batch [1]. That's not really the case though. The inner loop of write_cache_page() only ever locks one page at a time, either directly via the unlock_page() on L2338 (those goto's are amazing) or indirectly via (*writepage)() on L2359. So there's no deadlock potential there because unlocking any previously locked page(s) doesn't depend on obtaining the lock for another page. Unless I've missed something? >> The only deadlock prevention convention that I see is the convention of >> locking the pages in order of ascending address. That only helps if >> everything does it that way, and migrate code definitely does not. >> However...I thought that up until now, at least, the migrate code relied >> on trylock (which can fail, and so migration can fail, too), to avoid >> deadlock. Is that changing somehow, I didn't see it? > > The trylock is used by async mode which does try to avoid blocking. > But sync mode does use lock. The current implementation of migration > does migrate one page at a time, so it is not a problem. > >> >> >> [1] https://elixir.bootlin.com/linux/latest/source/mm/page-writeback.c#L2296 >> >> thanks, >> >> -- >> John Hubbard >> NVIDIA >> >> > The simplest solution is to batch page migration only if mode == >> > MIGRATE_ASYNC. Then we may consider to fall back to non-batch mode if >> > mode != MIGRATE_ASYNC and trylock page fails. >> > >> >>
