linux-kernel - Re: [PATCH v2 5/9] iomap: Support arbitrarily many blocks per page

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Date:   Tue, 22 Sep 2020 18:05:26 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Qian Cai <cai@...hat.com>
Cc:     linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-nvdimm@...ts.01.org, linux-kernel@...r.kernel.org,
        Dave Kleikamp <shaggy@...nel.org>,
        jfs-discussion@...ts.sourceforge.net,
        Dave Chinner <dchinner@...hat.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        linux-next@...r.kernel.org
Subject: Re: [PATCH v2 5/9] iomap: Support arbitrarily many blocks per page

On Tue, Sep 22, 2020 at 12:23:45PM -0400, Qian Cai wrote:
> On Fri, 2020-09-11 at 00:47 +0100, Matthew Wilcox (Oracle) wrote:
> > Size the uptodate array dynamically to support larger pages in the
> > page cache.  With a 64kB page, we're only saving 8 bytes per page today,
> > but with a 2MB maximum page size, we'd have to allocate more than 4kB
> > per page.  Add a few debugging assertions.
> > 
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> > Reviewed-by: Dave Chinner <dchinner@...hat.com>
> 
> Some syscall fuzzing will trigger this on powerpc:
> 
> .config: https://gitlab.com/cailca/linux-mm/-/blob/master/powerpc.config
> 
> [ 8805.895344][T445431] WARNING: CPU: 61 PID: 445431 at fs/iomap/buffered-io.c:78 iomap_page_release+0x250/0x270

Well, I'm glad it triggered.  That warning is:
        WARN_ON_ONCE(bitmap_full(iop->uptodate, nr_blocks) !=
                        PageUptodate(page));
so there was definitely a problem of some kind.

truncate_cleanup_page() calls
do_invalidatepage() calls
iomap_invalidatepage() calls
iomap_page_release()

Is this the first warning?  I'm wondering if maybe there was an I/O error
earlier which caused PageUptodate to get cleared again.  If it's easy to
reproduce, perhaps you could try something like this?

+void dump_iomap_page(struct page *page, const char *reason)
+{
+       struct iomap_page *iop = to_iomap_page(page);
+       unsigned int nr_blocks = i_blocks_per_page(page->mapping->host, page);
+
+       dump_page(page, reason);
+       if (iop)
+               printk("iop:reads %d writes %d uptodate %*pb\n",
+                               atomic_read(&iop->read_bytes_pending),
+                               atomic_read(&iop->write_bytes_pending),
+                               nr_blocks, iop->uptodate);
+       else
+               printk("iop:none\n");
+}

and then do something like:

	if (bitmap_full(iop->uptodate, nr_blocks) != PageUptodate(page))
		dump_iomap_page(page, NULL);