On Thu, 18 Jun 2009, Michael Tokarev wrote:
David Rientjes wrote:On Thu, 18 Jun 2009, Michael Tokarev wrote:We have the same issue - I replied to Justin's initial email with exactly
Not sure if you're referring to the bugzilla entry or Justin's reportedhttp://bugzilla.kernel.org/show_bug.cgi?id=13518Does not look similar.
I repeated the issue here. The slab which is growing here is buffer_head.
It's growing slowly -- right now, after ~5 minutes of constant writes over
nfs, its size is 428423 objects, growing at about 5000 objects/minute
When stopping writing, the cache shrinks slowly back to an acceptable
size, probably when the data gets actually written to disk.
issue. Justin's issue is actually allocating a skbuff_head_cache slab while
the system is oom.
the same trace as him. I didn't see your reply up until today, -- the one
you're referring to below.
If it's the exact same trace, then the page allocation failure is occurring as the result of slab's growth of the skbuff_head_cache cache, not buffer_head.
So it appears as though the issue you're raising is that buffer_head is consuming far too much memory, which causes the system to be oom when attempting a GFP_ATOMIC allocation for skbuff_head_cache and is otherwise unseen with alloc_buffer_head() because it is allowed to invoke direct reclaim:
$ grep -r alloc_buffer_head\( fs/*
fs/buffer.c: bh = alloc_buffer_head(GFP_NOFS);
fs/buffer.c:struct buffer_head *alloc_buffer_head(gfp_t gfp_flags)
fs/gfs2/log.c: bh = alloc_buffer_head(GFP_NOFS | __GFP_NOFAIL);
fs/jbd/journal.c: new_bh = alloc_buffer_head(GFP_NOFS|__GFP_NOFAIL);
fs/jbd2/journal.c: new_bh = alloc_buffer_head(GFP_NOFS|__GFP_NOFAIL);
As far as I can see, the warning itself, while harmless, indicates some
deeper problem. Namely, we shouldn't have an OOM condition - the system
is doing nothing but NFS, there's only one NFS client which writes single
large file, the system has 2GB (or 4Gb on another machine) RAM. It should
not OOM to start with.
Thanks to the page allocation failure that Justin posted earlier, which shows the state of the available system memory, it shows that the machine truly is oom. You seem to have isolated that to an enormous amount of buffer_head slab, which is a good start.
Well, there ARE side-effects actually. When the issue happens, the I/O
over NFS slows down to almost zero bytes/sec for some while, and resumes
slowly after about half a minute - sometimes faster, sometimes slower.
Again, the warning itself is harmless, but it shows a deeper issue. I
don't think it's wise to ignore the sympthom -- the actual cause should
be fixed instead. I think.
Since the GFP_ATOMIC allocation cannot trigger reclaim itself, it must rely on other allocations or background writeout to free the memory and this will be considerably slower than a blocking allocation. The page allocation failure messages from Justin's post indicate there are 0 pages under writeback at the time of oom yet ZONE_NORMAL has reclaimable memory; this is the result of the nonblocking allocation.