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From:   Miklos Szeredi <miklos@szeredi.hu>
Date:   Mon, 13 Jul 2020 18:14:16 +0200
Message-ID: <CAJfpegvU2JQcNM+0mcMPk-_e==RcT0xjqYUHCTzx3g0oCw6RiA@mail.gmail.com>
Subject: Re: [PATCH] fuse_writepages_fill() optimization to avoid WARN_ON in tree_insert
To:     Vasily Averin <vvs@virtuozzo.com>
Cc:     linux-fsdevel@vger.kernel.org, Maxim Patlasov <maximvp@gmail.com>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Jul 13, 2020 at 10:02 AM Vasily Averin <vvs@virtuozzo.com> wrote:
>
> On 7/11/20 7:01 AM, Miklos Szeredi wrote:
> > On Thu, Jun 25, 2020 at 11:02 AM Vasily Averin <vvs@virtuozzo.com> wrote:
> >>
> >> In current implementation fuse_writepages_fill() tries to share the code:
> >> for new wpa it calls tree_insert() with num_pages = 0
> >> then switches to common code used non-modified num_pages
> >> and increments it at the very end.
> >>
> >> Though it triggers WARN_ON(!wpa->ia.ap.num_pages) in tree_insert()
> >>  WARNING: CPU: 1 PID: 17211 at fs/fuse/file.c:1728 tree_insert+0xab/0xc0 [fuse]
> >>  RIP: 0010:tree_insert+0xab/0xc0 [fuse]
> >>  Call Trace:
> >>   fuse_writepages_fill+0x5da/0x6a0 [fuse]
> >>   write_cache_pages+0x171/0x470
> >>   fuse_writepages+0x8a/0x100 [fuse]
> >>   do_writepages+0x43/0xe0
> >>
> >> This patch re-works fuse_writepages_fill() to call tree_insert()
> >> with num_pages = 1 and avoids its subsequent increment and
> >> an extra spin_lock(&fi->lock) for newly added wpa.
> >
> > Looks good.  However, I don't like the way fuse_writepage_in_flight()
> > is silently changed to insert page into the rb_tree.  Also the
> > insertion can be merged with the search for in-flight and be done
> > unconditionally to simplify the logic.  See attached patch.
>
> Your patch looks correct for me except 2 things:

Thanks for reviewing.

> 1) you have lost "data->wpa = NULL;" when fuse_writepage_add() returns false.

This is intentional, because this is in the !data->wpa branch.

> 2) in the same case old code did not set data->orig_pages[ap->num_pages] = page;

That is also intentional, in this case the origi_pages[0] is either
overwritten with the next page or discarded due to data->wpa being
NULL.

I'll write these up in the patch header.

Thanks,
Miklos