forcing write-back from FS - again

Artem Bityutskiy
2007-10-21T20:19:58Z

Hi Andrew,

some time ago we were talking about doing write-back from inside a file-system
(http://marc.info/?l=linux-kernel&m=119097117713616&w=2). You said that I'm not
the only person who needs this, because the same thing is needed for delayed
allocation.

The problem is that if we initiate write-back from prepare_write() and we are
having a dirty page lock, we deadlock in write_cache_pages() which tries to
lock the same page.

You suggested to enhance struct writeback_control and put page that should be
skipped.

I tried something like

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -61,6 +61,7 @@ struct writeback_control {
unsigned for_reclaim:1; /* Invoked from the page allocator */
unsigned for_writepages:1; /* This is a writepages() call */
unsigned range_cyclic:1; /* range_start is cyclic */
+ struct page *skip_pg; /* do not write this page back */

void *fs_private; /* For use by ->writepages() */
};

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -641,6 +641,9 @@ retry:
for (i = 0; i < nr_pages; i++) {
struct page *page = pvec.pages[i];

+ if (unlikely(page == wbc->skip_pg))
+ continue;
+
/*
* At this point we hold neither mapping->tree_lock nor
* lock on the page itself: the page may be truncated

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).

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).

I'd appreciate any suggestions. Thanks!