Re: [RFC 2/6] mm/migrate_pages: split unmap_and_move() to _unmap() and _move()

[Yang Shi]

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
>