On Tue, Sep 27, 2022 at 6:56 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote: > > On 9/27/22 18:49, Yang Shi wrote: > > On Tue, Sep 27, 2022 at 6:45 PM John Hubbard <jhubbard@xxxxxxxxxx> wrote: > >> > >> On 9/27/22 18:41, Huang, Ying wrote: > >>>>>> 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? > >>> > >>> Yes. This is my understanding too after checking ext4_writepage(). > >>> > >> > >> Yes, I missed the ".writepage() shall unlock the page" design point. Now > >> it seems much more reasonable and safer. :) > > > > .writepage is deprecated (see > > https://lore.kernel.org/linux-fsdevel/20220719041311.709250-1-hch@xxxxxx/), > > write back actually uses .writepages. > > write_cache_pages() seems to directly call it, though: > > generic_writepages() > write_cache_pages(__writepage) > __writepage() > mapping->a_ops->writepage(page, wbc) > > So it seems like it's still alive and well. And in any case, it is definitely > passing one page at a time from write_cache_pages(), right? IIRC, the writeback may not call generic_writepages. On my ext4 filesystem, the writeback call stack looks like: @[ ext4_writepages+1 do_writepages+191 __writeback_single_inode+65 writeback_sb_inodes+477 __writeback_inodes_wb+76 wb_writeback+457 wb_workfn+680 process_one_work+485 worker_thread+80 kthread+231 ret_from_fork+34 ]: 2 > > > thanks, > > -- > John Hubbard > NVIDIA >
