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? thanks, -- John Hubbard NVIDIA
