On Mon, 2016-08-29 at 10:28 -0700, Linus Torvalds wrote: > > On Mon, Aug 29, 2016 at 7:52 AM, Olaf Hering <olaf@xxxxxxxxx> wrote: > > > > > > Today I noticed the nfsserver was disabled, probably since months already. > > Starting it gives a OOM, not sure if this is new with 4.7+. > > That's not an oom, that's just an allocation failure. > > And with order-4, that's actually pretty normal. Nobody should use > order-4 (that's 16 contiguous pages, fragmentation can easily make > that hard - *much* harder than the small order-2 or order-2 cases that > we should largely be able to rely on). > > In fact, people who do multi-order allocations should always have a > fallback, and use __GFP_NOWARN. > > > > > [93348.306406] Call Trace: > > [93348.306490] [<ffffffff81198cef>] __alloc_pages_slowpath+0x1af/0xa10 > > [93348.306501] [<ffffffff811997a0>] __alloc_pages_nodemask+0x250/0x290 > > [93348.306511] [<ffffffff811f1c3d>] cache_grow_begin+0x8d/0x540 > > [93348.306520] [<ffffffff811f23d1>] fallback_alloc+0x161/0x200 > > [93348.306530] [<ffffffff811f43f2>] __kmalloc+0x1d2/0x570 > > [93348.306589] [<ffffffffa08f025a>] nfsd_reply_cache_init+0xaa/0x110 [nfsd] > > Hmm. That's kmalloc itself falling back after already failing to grow > the slab cache earlier (the earlier allocations *were* done with > NOWARN afaik). > > It does look like nfsdstarts out by allocating the hash table with one > single fairly big allocation, and has no fallback position. > > I suspect the code expects to be started at boot time, when this just > isn't an issue. The fact that you loaded the nfsd kernel module with > memory already fragmented after heavy use is likely why nobody else > has seen this. > > Adding the nfsd people to the cc, because just from a robustness > standpoint I suspect it would be better if the code did something like > > (a) shrink the hash table if the allocation fails (we've got some > examples of that elsewhere) > > or > > (b) fall back on a vmalloc allocation (that's certainly the simpler model) > > We do have a "kvfree()" helper function for the "free either a kmalloc > or vmalloc allocation" but we don't actually have a good helper > pattern for the allocation side. People just do it by hand, at least > partly because we have so many different ways to allocate things - > zeroing, non-zeroing, node-specific or not, atomic or not (atomic > cannot fall back to vmalloc, obviously) etc etc. > > Bruce, Jeff, comments? > > Linus Yeah, that makes total sense. Hmm...we _do_ already auto-size the hash at init time already, so shrinking it downward and retrying if the allocation fails wouldn't be hard to do. Maybe I can just cut it in half and throw a pr_warn to tell the admin in that case. In any case...I'll take a look at how we can improve it. Thanks for the heads-up! -- Jeff Layton <jlayton@xxxxxxxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
