fs: __generic_file_splice_read retry lookup on AOP_TRUNCATED_PAGE

fs: __generic_file_splice_read retry lookup on AOP_TRUNCATED_PAGE

During testing, I discovered that __generic_file_splice_read() returns
0 (EOF) when aops->readpage fails with AOP_TRUNCATED_PAGE on the first
page of a single/multi-page splice read operation. This EOF return code
causes the userspace test to (correctly) report a zero-length read error
when it was expecting otherwise.

The current strategy of returning a partial non-zero read when ->readpage
returns AOP_TRUNCATED_PAGE works only when the failed page is not the
first of the lot being processed.

This patch attempts to retry lookup and call ->readpage again on pages
that had previously failed with AOP_TRUNCATED_PAGE. With this patch, my
tests pass and I haven't noticed any unwanted side effects.

This version removes the thrice-retry loop and instead indefinitely
retries lookups on AOP_TRUNCATED_PAGE errors from ->readpage. This
behavior is now similar to do_generic_file_read().

Signed-off-by: Abhi Das <adas@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Bob Peterson <rpeterso@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>