Re: [PATCH] iomap: Set all uptodate bits for an Uptodate page

[Gao Xiang <hsiangkao@redhat.com>]

On Thu, Sep 24, 2020 at 02:59:00PM +0100, Matthew Wilcox wrote:
> On Thu, Sep 24, 2020 at 09:12:35AM -0400, Brian Foster wrote:
> > On Thu, Sep 24, 2020 at 01:56:08PM +0100, Matthew Wilcox (Oracle) wrote:
> > > For filesystems with block size < page size, we need to set all the
> > > per-block uptodate bits if the page was already uptodate at the time
> > > we create the per-block metadata.  This can happen if the page is
> > > invalidated (eg by a write to drop_caches) but ultimately not removed
> > > from the page cache.
> > > 
> > > This is a data corruption issue as page writeback skips blocks which
> > > are marked !uptodate.
> > 
> > Thanks. Based on my testing of clearing PageUptodate here I suspect this
> > will similarly prevent the problem, but I'll give this a test
> > nonetheless. 
> > 
> > I am a little curious why we'd prefer to fill the iop here rather than
> > just clear the page state if the iop data has been released. If the page
> > is partially uptodate, then we end up having to re-read the page
> > anyways, right? OTOH, I guess this behavior is more consistent with page
> > size == block size filesystems where iop wouldn't exist and we just go
> > by page state, so perhaps that makes more sense.
> 
> Well, it's _true_ ... the PageUptodate bit means that every byte in this
> page is at least as new as every byte on storage.  There's no need to
> re-read it, which is what we'll do if we ClearPageUptodate.
> 

Agreed, Pagetodate(page) means the whole page content is available now,
see create_page_buffers() -> create_empty_buffers() and try_to_free_buffers()
(much like .releasepage()) in buffer head approach.

Reviewed-by: Gao Xiang <hsiangkao@redhat.com>

Thanks,
Gao Xiang