linux-kernel - Re: [PATCH v4 03/11] fs/9p: Consolidate file operations and add readahead and writeback

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Message-ID: <CAFkjPTm909jFaEnpmSMBu-6uZnPBVyU_KqMFzWCwbDopT4jCAA@mail.gmail.com>
Date:   Sat, 18 Feb 2023 10:17:20 -0600
From:   Eric Van Hensbergen <ericvh@...il.com>
To:     asmadeus@...ewreck.org
Cc:     Eric Van Hensbergen <ericvh@...nel.org>,
        v9fs-developer@...ts.sourceforge.net, rminnich@...il.com,
        lucho@...kov.net, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux_oss@...debyte.com
Subject: Re: [PATCH v4 03/11] fs/9p: Consolidate file operations and add
 readahead and writeback

On Sat, Feb 18, 2023 at 3:25 AM <asmadeus@...ewreck.org> wrote:
>
> Ok so this bugged me to no end; that seems to be because we use the same
> v9fs_dir_release for v9fs_file_operations's .release and not just
> v9fs_dir_operations... So it's to be expected we'll get files here.
>
> At this point I'd suggest to use two functions, but that's probably
> overdoing it.
> Let's check S_ISREG(inode->i_mode) instead of fid->qid though; it
> shouldn't make any difference but that's what you use in other parts of
> the code and it will be easier to understand for people familiar with
> the vfs.
>

I can rename the function as part of the patch since it would be a bit
more accurate,
but then it is still in vfs_dir.  I think there did used to be two
functions but there
was so much overlap we collapsed into one.

>
> > diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c
> > index 33e521c60e2c..8ffa6631b1fd 100644
> > --- a/fs/9p/vfs_inode.c
> > +++ b/fs/9p/vfs_inode.c
> > @@ -219,6 +219,35 @@ v9fs_blank_wstat(struct p9_wstat *wstat)
> >       wstat->extension = NULL;
> >  }
> >
> > +/**
> > + * v9fs_flush_inode_writeback - writeback any data associated with inode
> > + * @inode: inode to writeback
> > + *
> > + * This is used to make sure anything that needs to be written
> > + * to server gets flushed before we do certain operations (setattr, getattr, close)
> > + *
> > + */
> > +
> > +int v9fs_flush_inode_writeback(struct inode *inode)
> > +{
> > +     struct writeback_control wbc = {
> > +             .nr_to_write = LONG_MAX,
> > +             .sync_mode = WB_SYNC_ALL,
> > +             .range_start = 0,
> > +             .range_end = -1,
> > +     };
> > +
> > +     int retval = filemap_fdatawrite_wbc(inode->i_mapping, &wbc);
>
> Hmm, that function only starts the writeback, but doesn't wait for it.
>
> Wasn't the point to replace 'filemap_write_and_wait' with
> v9fs_flush_inode_writeback?
> I don't think it's a good idea to remove the wait before setattrs and
> the like; if you don't want to wait on close()'s release (but we
> probably should too) perhaps split this in two?
>

I had thought that this is what it does, of course I could just be getting
lucky.  The filemap_fdatawrite_wbc doesn't say anything about whether
WBC_SYNC_ALL forces a wait, but the next function (__filemap_fdatawrite_range)
does: (it it calls filemap_fdatawrite_wbc)

* If sync_mode is WB_SYNC_ALL then this is a "data integrity" operation, as
* opposed to a regular memory cleansing writeback. The difference between
* these two operations is that if a dirty page/buffer is encountered, it must
* be waited upon, and not just skipped over.

So I think we are good?  Happy to use a different function if it makes sense,
but this was the one that seemed to trigger the correct behavior.

       -eric