linux-kernel - Re: [PATCH v2 19/28] iomap: Convert __iomap_zero

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Message-ID: <YbulCVh8xwwRKAN3@casper.infradead.org>
Date:   Thu, 16 Dec 2021 20:43:53 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v2 19/28] iomap: Convert __iomap_zero_iter to use a folio

On Thu, Dec 16, 2021 at 11:36:14AM -0800, Darrick J. Wong wrote:
> > 
> > +       /* gfs2 does not support large folios yet */
> > +       if (len > PAGE_SIZE)
> > +               len = PAGE_SIZE;
> 
> This is awkward -- gfs2 doesn't set the mapping flag to indicate that it
> supports large folios, so it should never be asked to deal with more
> than a page at a time.  Shouldn't iomap_write_begin clamp its len
> argument to PAGE_SIZE at the start if the mapping doesn't have the large
> folios flag set?

You're right, this is awkward.  And it's a bit of a beartrap for
another filesystem that wants to implement ->prepare_page in the
future.

diff --git a/fs/gfs2/bmap.c b/fs/gfs2/bmap.c
index 9270db17c435..d67108489148 100644
--- a/fs/gfs2/bmap.c
+++ b/fs/gfs2/bmap.c
@@ -968,9 +968,6 @@ static int gfs2_iomap_page_prepare(struct inode *inode, loff_t pos,
        struct gfs2_sbd *sdp = GFS2_SB(inode);
        unsigned int blocks;

-       /* gfs2 does not support large folios yet */
-       if (len > PAGE_SIZE)
-               len = PAGE_SIZE;
        blocks = ((pos & blockmask) + len + blockmask) >> inode->i_blkbits;
        return gfs2_trans_begin(sdp, RES_DINODE + blocks, 0);
 }
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 1a9e897ee25a..b1ded5204d1c 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -619,6 +619,9 @@ static int iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
        if (fatal_signal_pending(current))
                return -EINTR;
 
+       if (!mapping_large_folio_support(iter->inode->i_mapping))
+               len = min_t(size_t, len, PAGE_SIZE - offset_in_page(pos));
+
        if (page_ops && page_ops->page_prepare) {
                status = page_ops->page_prepare(iter->inode, pos, len);
                if (status)