[PATCH v2 3/3] netfs: fix test for whether we can skip read when writing beyond EOF

[Matthew Wilcox]

On Thu, Jun 17, 2021 at 09:24:27AM +0100, David Howells wrote:
> From: Jeff Layton <jlayton at kernel.org>
> 
> It's not sufficient to skip reading when the pos is beyond the EOF.
> There may be data at the head of the page that we need to fill in
> before the write.
> 
> Add a new helper function that corrects and clarifies the logic of
> when we can skip reads, and have it only zero out the part of the page
> that won't have data copied in for the write.
> 
> Finally, don't set the page Uptodate after zeroing. It's not up to date
> since the write data won't have been copied in yet.
> 
> [DH made the following changes:
> 
>  - Prefixed the new function with "netfs_".
> 
>  - Don't call zero_user_segments() for a full-page write.
> 
>  - Altered the beyond-last-page check to avoid a DIV instruction and got
>    rid of then-redundant zero-length file check.
> ]
> 
> Fixes: e1b1240c1ff5f ("netfs: Add write_begin helper")
> Reported-by: Andrew W Elble <aweits at rit.edu>
> Signed-off-by: Jeff Layton <jlayton at kernel.org>
> Signed-off-by: David Howells <dhowells at redhat.com>
> cc: ceph-devel at vger.kernel.org
> Link: https://lore.kernel.org/r/20210613233345.113565-1-jlayton@kernel.org/
> Link: https://lore.kernel.org/r/162367683365.460125.4467036947364047314.stgit@warthog.procyon.org.uk/ # v1

Reviewed-by: Matthew Wilcox (Oracle) <willy at infradead.org>