linux-kernel - Re: [PATCH v2 1/2] fuse: Fix possible deadlock when writing back dirty pages

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Date:   Wed, 14 Apr 2021 15:20:04 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Peng Tao <bergwolf@...il.com>
Cc:     Baolin Wang <baolin.wang@...ux.alibaba.com>,
        Peng Tao <tao.peng@...ux.alibaba.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] fuse: Fix possible deadlock when writing back
 dirty pages

On Wed, Apr 14, 2021 at 2:22 PM Peng Tao <bergwolf@...il.com> wrote:
>

> > --- a/fs/fuse/file.c
> > +++ b/fs/fuse/file.c
> > @@ -1117,17 +1117,12 @@ static ssize_t fuse_send_write_pages(str
> >       count = ia->write.out.size;
> >       for (i = 0; i < ap->num_pages; i++) {
> >               struct page *page = ap->pages[i];
> > +             bool page_locked = ap->page_locked && (i == ap->num_pages - 1);
> Any reason for just handling the last locked page in the page array?
> To be specific, it look like the first page in the array can also be
> partial dirty and locked?

In that case the first partial page will be locked, and it'll break
out of the loop...

> >
> > -             if (!err && !offset && count >= PAGE_SIZE)
> > -                     SetPageUptodate(page);
> > -
> > -             if (count > PAGE_SIZE - offset)
> > -                     count -= PAGE_SIZE - offset;
> > -             else
> > -                     count = 0;
> > -             offset = 0;
> > -
> > -             unlock_page(page);
> > +             if (err)
> > +                     ClearPageUptodate(page);
> > +             if (page_locked)
> > +                     unlock_page(page);
> >               put_page(page);
> >       }
> >
> > @@ -1191,6 +1186,16 @@ static ssize_t fuse_fill_write_pages(str
> >               if (offset == PAGE_SIZE)
> >                       offset = 0;
> >
> > +             /* If we copied full page, mark it uptodate */
> > +             if (tmp == PAGE_SIZE)
> > +                     SetPageUptodate(page);
> > +
> > +             if (PageUptodate(page)) {
> > +                     unlock_page(page);
> > +             } else {
> > +                     ap->page_locked = true;
> > +                     break;

... here, and send it as a separate WRITE request.

So the multi-page case with a partial & non-uptodate head page will
always result in the write request being split into two (even if
there's no partial tail page).

Thanks,
Miklos