Re: forcing write-back from FS - again

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Andrew Morton wrote:
but it does not dot actually work, because if we have two processes forcing write-back from write_page(), they will mutually deadlock (A waits in write_cache_pages() on a page B has locked, B waits on inode or page A has locked).

Yeah, I was just thinking that as I read this ;)
So this way is not ok, do you have any other ideas?

We could mark page clean temporarily before doing write-back, and mark it dirty again, but this seems to be inefficient (although I'm not sure, need to dig these functions deeper, but they _seem_ to traverse the radix tree and change tags, so marking one page dirty may need to change many tags, but again, I did not really dig tis yet).

We could just skip locked pages altogether in writeback.  Perhaps in
WB_SYNC_NONE mode, or perhaps add a new flag in writeback_control to select
this behaviour.

Yeah, certanly not WB_SYNC_ALL, because this is a deadlocky - the process which forces write-back from the ->prepare_write() is having page X locked, pdflush may have some inode A locked and sleep on page X, while the FS would sleep on inode A.

It _should_ be the case that the number of locked-and-dirty pages which
writeback encounters is very small, so skipping locked pages during
writeback-for-memory-flushing won't have any significant effect.  The first
step should be to add a new /proc/vmstat field to count these pages and
then do broad testing (especially on blocksize<pagesize filesystems) to
confirm the theory.

We'll still need to synchronously lock the page in
writeback-for-data-integrity mode though.

Thanks for suggestion. It sounds as a separate big job to enhance existing WB_SYNC_NONE. I've just introduced new WB mode, and it seems to work fine:

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
@@ -28,6 +28,7 @@ static inline int task_is_pdflush(struct task_struct *task)
  */
 enum writeback_sync_modes {
        WB_SYNC_NONE,   /* Don't wait on anything */
+       WB_SYNC_NONE_PG,/* Don't wait on anything, don't touch locked pages */
        WB_SYNC_ALL,    /* Wait on every mapping */
        WB_SYNC_HOLD,   /* Hold the inode on sb_dirty for sys_sync() */
 };
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
@@ -641,6 +641,10 @@ retry:
                for (i = 0; i < nr_pages; i++) {
                        struct page *page = pvec.pages[i];

+                       if (wbc->sync_mode == WB_SYNC_NONE_PG &&
+                           PageLocked(page))
+                               continue;
+

My only concern is - what if the page we skipped because of WB_SYNC_NONE_PG will somehow loose its dirty TAG and will never be written-back? But it is because of my poor knowledge of Linux MM internals. Could you please comment on this?

Thanks!

--
Best Regards,
Artem Bityutskiy (Артём Битюцкий)

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