linux-kernel - Re: [PATCH 3/3] Revert "mm: swap: check if swap backing device is congested or not"

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Date:   Tue, 10 Aug 2021 14:30:17 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Huang Ying <ying.huang@...el.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] Revert "mm: swap: check if swap backing device is
 congested or not"

On Tue, Aug 10, 2021 at 2:08 PM Hugh Dickins <hughd@...gle.com> wrote:
>
> On Tue, 10 Aug 2021, Yang Shi wrote:
>
> > Due to the change about how block layer detects congestion the
> > justification of commit 8fd2e0b505d1 ("mm: swap: check if swap backing device
> > is congested or not") doesn't stand anymore, so the commit could be just
> > reverted in order to solve the race reported by commit 2efa33fc7f6e ("mm/shmem:
> > fix shmem_swapin() race with swapoff"). The fix was reverted by the
> > previous patch.
> >
> > Suggested-by: Hugh Dickins <hughd@...gle.com>
> > Cc: "Huang, Ying" <ying.huang@...el.com>
> > Cc: Miaohe Lin <linmiaohe@...wei.com>
> > Signed-off-by: Yang Shi <shy828301@...il.com>
>
> Thanks for doing these,
> Acked-by: Hugh Dickins <hughd@...gle.com>
> to all three.
>
> I guess 1/3 will vanish now, since what it's reverting has not yet gone
> forward to Linus: it would have been nice for 2/3 to have mentioned that
> what it's reverting is kindof buggy: but then you'd have to explain the
> "kindof", and that takes more words than it's worth - maybe a lore Link
> to the discussion?

It is fine, I think this one is the best elaborated the problem:
https://lore.kernel.org/linux-mm/24187e5e-069-9f3f-cefe-39ac70783753@google.com/

>
> But you've done the work that I've not yet got around to:
> so thank you, and ignore my gripes.
>
> Hugh
>
> > ---
> >  mm/swap_state.c | 7 -------
> >  1 file changed, 7 deletions(-)
> >
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index 1a29b4f98208..8d4104242100 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -628,13 +628,6 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
> >       if (!mask)
> >               goto skip;
> >
> > -     /* Test swap type to make sure the dereference is safe */
> > -     if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
> > -             struct inode *inode = si->swap_file->f_mapping->host;
> > -             if (inode_read_congested(inode))
> > -                     goto skip;
> > -     }
> > -
> >       do_poll = false;
> >       /* Read a page_cluster sized and aligned cluster around offset. */
> >       start_offset = offset & ~mask;
> > --
> > 2.26.2