RE: [PATCH v3] ext4: fix data integrity sync in ordered mode

[Namjae Jeon <namjae.jeon@samsung.com>]

> 
> On Wed 07-05-14 14:15:24, Namjae Jeon wrote:
> > When we perform a data integrity sync we tag all the dirty pages with
> > PAGECACHE_TAG_TOWRITE at start of ext4_da_writepages.
> > Later we check for this tag in write_cache_pages_da and creates a
> > struct mpage_da_data containing contiguously indexed pages tagged with this
> > tag and sync these pages with a call to mpage_da_map_and_submit.
> > This process is done in while loop until all the PAGECACHE_TAG_TOWRITE pages
> > are synced. We also do journal start and stop in each iteration.
> > journal_stop could initiate journal commit which would call ext4_writepage
> > which in turn will call ext4_bio_write_page even for delayed OR unwritten
> > buffers. When ext4_bio_write_page is called for such buffers, even though it
> > does not sync them but it clears the PAGECACHE_TAG_TOWRITE of the corresponding
> > page and hence these pages are also not synced by the currently running data
> > integrity sync. We will end up with dirty pages although sync is completed.
> >
> > This could cause a potential data loss when the sync call is followed by a
> > truncate_pagecache call, which is exactly the case in collapse_range.
> > (It will cause generic/127 failure in xfstests)
> >
> > To avoid this issue, we can use set_page_writeback_keepwrite instead of
> > set_page_writeback, which doesn't clear TOWRITE tag.
> >
> > Cc: Jan Kara <jack@suse.cz>
> > Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
> > Signed-off-by: Ashish Sangwan <a.sangwan@samsung.com>
>   The patch looks good. You can add:
> Reviewed-by: Jan Kara <jack@suse.cz>
> 
> BTW you should also cc linux-mm@kvack.org with this patch since it touches
> the mm code.
Okay. Thanks for your review!
> 
> 									Honza