Re: [fuse-devel] [PATCH 14/14] mm: Account for WRITEBACK_TEMP inbalance_dirty_pages

From: Maxim V. Patlasov
Date: Thu Apr 25 2013 - 12:19:14 EST


04/25/2013 07:49 PM, Miklos Szeredi ÐÐÑÐÑ:
On Thu, Apr 25, 2013 at 4:29 PM, Maxim V. Patlasov
<mpatlasov@xxxxxxxxxxxxx> wrote:
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 0713bfb..c47bcd4 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -1235,7 +1235,8 @@ static void balance_dirty_pages(struct address_space
nr_reclaimable = global_page_state(NR_FILE_DIRTY) +

- nr_dirty = nr_reclaimable +
+ nr_dirty = nr_reclaimable +
global_page_state(NR_WRITEBACK) +
+ global_page_state(NR_WRITEBACK_TEMP);
global_dirty_limits(&background_thresh, &dirty_thresh);

Please drop this patch. As we discussed in LSF/MM, the fix above is correct,
but it's not enough: we also need to ensure disregard of NR_WRITEBACK_TEMP
when balance_dirty_pages() is called from fuse daemon. I'll send a separate
patch-set soon.
Please elaborate. From a technical perspective "fuse daemon" is very
hard to define, so anything that relies on whether something came from
the fuse daemon or not is conceptually broken.

As Mel Gorman pointed out, fuse daemon diving into balance_dirty_pages should not kick flusher judging on NR_WRITEBACK_TEMP. Essentially, all we need in balance_dirty_pages is:

if (I'm not fuse daemon)
nr_dirty += global_page_state(NR_WRITEBACK_TEMP);

The way how to identify fuse daemon was not thoroughly scrutinized during LSF/MM. Firstly, I thought it would be enough to set a per-process flag handling fuse device open. But now I understand that fuse daemon may be quite a complicated multi-threaded multi-process construction. I'm going to add new FUSE_NOTIFY to allow fuse daemon decide when it works on behalf of draining writeout-s. Having in mind that fuse-lib is multi-threaded, I'm also going to inherit the flag on copy_process(). Does it make sense for you?

Also, another patch will put this ad-hoc FUSE_NOTIFY under fusermount control. This will prevent malicious unprivileged fuse mounts from setting the flag for malicious purposes.

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