Re: [PATCH] mm/filemap: Use filemap_read_page in filemap_fault

[Matthew Wilcox <willy@infradead.org>]

On Wed, Mar 03, 2021 at 01:51:42PM -0800, Andrew Morton wrote:
> On Wed, 3 Mar 2021 20:57:36 +0000 Matthew Wilcox <willy@infradead.org> wrote:
> 
> > On Wed, Mar 03, 2021 at 12:12:53PM -0800, Andrew Morton wrote:
> > > On Wed, 3 Mar 2021 13:26:40 +0000 Matthew Wilcox <willy@infradead.org> wrote:
> > > 
> > > > But here's the thing ... invalidate_mapping_pages() doesn't
> > > > ClearPageUptodate.  The only places where we ClearPageUptodate is on an
> > > > I/O error.
> > > 
> > > yup.
> > > 
> > > > So ... as far as I can tell, the only way to hit this is:
> > > > 
> > > >  - Get an I/O error during the wait
> > > >  - Have another thread cause the page to be removed from the page cache
> > > >    (eg do direct I/O to the file) before this thread is run.
> > > > 
> > > > and the consequence to this change is that we have another attempt to
> > > > read the page instead of returning an error immediately.  I'm OK with
> > > > that unintentional change, although I think the previous behaviour was
> > > > also perfectly acceptable (after all, there was an I/O error while trying
> > > > to read this page).
> > > > 
> > > > Delving into the linux-fullhistory tree, this code was introduced by ...
> > > > 
> > > > commit 56f0d5fe6851037214a041a5cb4fc66199256544
> > > > Author: Andrew Morton <akpm@osdl.org>
> > > > Date:   Fri Jan 7 22:03:01 2005 -0800
> > > > 
> > > >     [PATCH] readpage-vs-invalidate fix
> > > > 
> > > >     A while ago we merged a patch which tried to solve a problem wherein a
> > > >     concurrent read() and invalidate_inode_pages() would cause the read() to
> > > >     return -EIO because invalidate cleared PageUptodate() at the wrong time.
> > > > 
> > > > We no longer clear PageUptodate, so I think this is stale code?  Perhaps
> > > > you could check with the original author ...
> > > 
> > > Which code do you think might be stale?  We need the !PageUptodate
> > > check to catch IO errors and we need the !page->mapping check to catch
> > > invalidates.  Am a bit confused.
> > 
> > I think the check of !page->mapping here:
> > 
> >         if (PageUptodate(page))
> >                 return 0;
> >         if (!page->mapping)     /* page truncated */
> >                 return AOP_TRUNCATED_PAGE;
> > 
> > is no longer needed.  If we didn't see an error, the page will be Uptodate,
> > regardless of whether it's been removed from the page cache.  If we did
> > see an error, it's OK to return -EIO, even if the page has been removed
> > from the page cache in the interim.
> 
> OK.
> 
> Checking page->mapping of an unlocked page seems meaningless anyway -
> what's to prevent it from being truncated just after we checked?

Originally it prevented the opposite race from being misinterpreted; we
didn't care if it was truncated after we saw it was Uptodate; we cared
if it was !Uptodate because it had been truncated.  Now, it's just pointless.