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SubjectRe: [PATCH] ext4: fix checking on nr_to_write
On Mon, 14 Oct 2013 19:34:59 +0200
Jan Kara <jack@suse.cz> wrote:

> On Mon 14-10-13 21:50:54, Ming Lei wrote:
> > On Mon, Oct 14, 2013 at 8:58 PM, Jan Kara <jack@suse.cz> wrote:
> > > Umm, I guess I see what are you pointing at. Thanks for catching that.
> > > mpage_process_page_bufs() always adds a buffer to mpd even if nr_to_write
> > > is already <= 0. But I would somewhat prefer not to call
> > > mpage_prepare_extent_to_map() at all when nr_to_write <= 0. So a patch
> > > like:
> > > ret = mpage_prepare_extent_to_map(&mpd);
> > > if (!ret) {
> > > - if (mpd.map.m_len)
> > > + if (mpd.map.m_len) {
> > > ret = mpage_map_and_submit_extent(handle, &mpd,
> > > &give_up_on_write);
> > > - else {
> > > + done = (wbc->nr_to_write <= 0);
> > > + } else {
> > >
> > > Should also fix your problem, am I right?
> >
> > I am afraid it can't, because we need to stop scanning page cache
> > if end of file is reached.
> That should be OK. mpage_process_page_bufs() won't add a buffer beyond
> EOF so we end the extent at EOF and next time we don't add anything to the
> extent. My change wouldn't change anything in this.

I mean wbc->nr_to_write may still be positive when mpage_prepare_extent_to_map()
returns zero, so your change will cause ext4_writepages not to break from the
loop, then write hangs, too crazy?

>
> > nr_to_write will become negative inside mpage_map_and_submit_extent(),
> > that is why I fix it inside mpage_prepare_extent_to_map().
> Yes, mpage_map_and_submit_extent() creates negative nr_to_write but only
> because mpage_prepare_extent_to_map() asked for mapping too big extent. But
> if I'm reading the code correctly we first ask for writing the extent of
> just the right size (nr_to_write becomes 0) but then ext4_writepages() asks
> mpage_prepare_extent_to_map() for another extent and it will create a single
> page extent for mapping before it finds out nr_to_write <= 0 and
> terminates. Am I understanding the problem correctly?

Your understanding is right, and I think it is a correct fix, because
we shouldn't add more pages to extent than nr_to_write.

>
> After thinking about it again, moving the condition in
> mpage_prepare_extent_to_map() as you did is probably better that what I
> suggested. Just move it more up in the loop - like after page->index > end
> condition. So that we don't unnecessarily lock the page etc.

Looks it makes sense, so how about below change?

--
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 32c04ab..c32b599 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2294,7 +2294,7 @@ static int mpage_prepare_extent_to_map(struct mpage_da_data *mpd)
{
struct address_space *mapping = mpd->inode->i_mapping;
struct pagevec pvec;
- unsigned int nr_pages;
+ unsigned int nr_pages, nr_added = 0;
pgoff_t index = mpd->first_page;
pgoff_t end = mpd->last_page;
int tag;
@@ -2330,6 +2330,18 @@ static int mpage_prepare_extent_to_map(struct mpage_da_data *mpd)
if (page->index > end)
goto out;

+ /*
+ * Accumulated enough dirty pages? This doesn't apply
+ * to WB_SYNC_ALL mode. For integrity sync we have to
+ * keep going because someone may be concurrently
+ * dirtying pages, and we might have synced a lot of
+ * newly appeared dirty pages, but have not synced all
+ * of the old dirty pages.
+ */
+ if (mpd->wbc->sync_mode == WB_SYNC_NONE &&
+ nr_added >= mpd->wbc->nr_to_write)
+ goto out;
+
/* If we can't merge this page, we are done. */
if (mpd->map.m_len > 0 && mpd->next_page != page->index)
goto out;
@@ -2364,19 +2376,7 @@ static int mpage_prepare_extent_to_map(struct mpage_da_data *mpd)
if (err <= 0)
goto out;
err = 0;
-
- /*
- * Accumulated enough dirty pages? This doesn't apply
- * to WB_SYNC_ALL mode. For integrity sync we have to
- * keep going because someone may be concurrently
- * dirtying pages, and we might have synced a lot of
- * newly appeared dirty pages, but have not synced all
- * of the old dirty pages.
- */
- if (mpd->wbc->sync_mode == WB_SYNC_NONE &&
- mpd->next_page - mpd->first_page >=
- mpd->wbc->nr_to_write)
- goto out;
+ nr_added++;
}
pagevec_release(&pvec);
cond_resched();


Thanks,
--
Ming Lei


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