Re: [PATCH v7 21/24] iomap: Restructure iomap_readpages_actor

[Darrick J. Wong]

On Fri, Feb 21, 2020 at 05:54:35PM -0800, Matthew Wilcox wrote:
> On Fri, Feb 21, 2020 at 04:44:25PM -0800, Darrick J. Wong wrote:
> > On Wed, Feb 19, 2020 at 01:01:00PM -0800, Matthew Wilcox wrote:
> > > From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> > > 
> > > By putting the 'have we reached the end of the page' condition at the end
> > > of the loop instead of the beginning, we can remove the 'submit the last
> > > page' code from iomap_readpages().  Also check that iomap_readpage_actor()
> > > didn't return 0, which would lead to an endless loop.
> > > 
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > > ---
> > >  fs/iomap/buffered-io.c | 32 ++++++++++++++++++--------------
> > >  1 file changed, 18 insertions(+), 14 deletions(-)
> > > 
> > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > > index cb3511eb152a..31899e6cb0f8 100644
> > > --- a/fs/iomap/buffered-io.c
> > > +++ b/fs/iomap/buffered-io.c
> > > @@ -400,15 +400,9 @@ iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
> > >  		void *data, struct iomap *iomap, struct iomap *srcmap)
> > >  {
> > >  	struct iomap_readpage_ctx *ctx = data;
> > > -	loff_t done, ret;
> > > -
> > > -	for (done = 0; done < length; done += ret) {
> > > -		if (ctx->cur_page && offset_in_page(pos + done) == 0) {
> > > -			if (!ctx->cur_page_in_bio)
> > > -				unlock_page(ctx->cur_page);
> > > -			put_page(ctx->cur_page);
> > > -			ctx->cur_page = NULL;
> > > -		}
> > > +	loff_t ret, done = 0;
> > > +
> > > +	while (done < length) {
> > >  		if (!ctx->cur_page) {
> > >  			ctx->cur_page = iomap_next_page(inode, ctx->pages,
> > >  					pos, length, &done);
> > > @@ -418,6 +412,20 @@ iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
> > >  		}
> > >  		ret = iomap_readpage_actor(inode, pos + done, length - done,
> > >  				ctx, iomap, srcmap);
> > > +		done += ret;
> > > +
> > > +		/* Keep working on a partial page */
> > > +		if (ret && offset_in_page(pos + done))
> > > +			continue;
> > > +
> > > +		if (!ctx->cur_page_in_bio)
> > > +			unlock_page(ctx->cur_page);
> > > +		put_page(ctx->cur_page);
> > > +		ctx->cur_page = NULL;
> > > +
> > > +		/* Don't loop forever if we made no progress */
> > > +		if (WARN_ON(!ret))
> > > +			break;
> > >  	}
> > >  
> > >  	return done;
> > > @@ -451,11 +459,7 @@ iomap_readpages(struct address_space *mapping, struct list_head *pages,
> > >  done:
> > >  	if (ctx.bio)
> > >  		submit_bio(ctx.bio);
> > > -	if (ctx.cur_page) {
> > > -		if (!ctx.cur_page_in_bio)
> > > -			unlock_page(ctx.cur_page);
> > > -		put_page(ctx.cur_page);
> > > -	}
> > > +	BUG_ON(ctx.cur_page);
> > 
> > Whoah, is the system totally unrecoverably hosed at this point?
> > 
> > I get that this /shouldn't/ happen, but should we somehow end up with a
> > page here, are we unable either to release it or even just leak it?  I'd
> > have thought a WARN_ON would be just fine here.
> 
> If we do find a page here, we don't actually know what to do with it.
> It might be (currently) locked, it might have the wrong refcount.
> Whatever is going on, it's probably better that we stop everything right
> here rather than allow things to go further and possibly present bad
> data to the application.  I mean, we could even be leaking the previous
> contents of this page to userspace.  Or maybe the future contents of a
> page which shouldn't be in the page cache any more, but userspace gets
> a mapping to it.
> 
> I'm not enthusiastic about putting in some code here to try to handle
> a "can't happen" case, since it's never going to be tested, and might
> end up causing more problems than it tries to solve.  Let's just stop.

Seeing how Linus (and others like myself) are a bit allergic to BUG
these days, could you add the first paragraph of the above justification
as a comment adjacent to the BUG_ON(), please? :)

--D