From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail.ilvokhin.com (mail.ilvokhin.com [178.62.254.231]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7809833345A for ; Wed, 20 May 2026 11:35:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=178.62.254.231 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779276953; cv=none; b=dIWVruT78O34dqdI1JvUIZkp3hqMA3lmhHGJcPtwwXlSMJAnbh2pHaKRCZ+nvVUVWC/1z2I/MGpXZgeBFuye3okMCxfaunChMZjgO6tDtej1ADd3TizIHtPfrEAg0audWI1nD3gkzLq8mtqFU0PDwpkzVG2PGfLS3v2kk0Bk21g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779276953; c=relaxed/simple; bh=8KKPz32/KDh27vrBJQFJll/6j9lwGtsaN8+UkQnOOhI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=uwKAR/mmp2YagyBEJo+NYoga8Szb9hEXWDZmTM6FfAVhR1o3fCFQdfLCJQ1hIKSKPYZx02i9laRHY8hkGgIEx61dSyY7uNTfJWZeuVjfRK5rQ1N1WXivXIBwyhqXhJZDr9xAnszoCqiUzUu2vENomDq0GcMdd5ALdCYSwTRYJts= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=ilvokhin.com; spf=pass smtp.mailfrom=ilvokhin.com; dkim=pass (1024-bit key) header.d=ilvokhin.com header.i=@ilvokhin.com header.b=XKbT3kpX; arc=none smtp.client-ip=178.62.254.231 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=ilvokhin.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=ilvokhin.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=ilvokhin.com header.i=@ilvokhin.com header.b="XKbT3kpX" Received: from shell.ilvokhin.com (shell.ilvokhin.com [138.68.190.75]) (Authenticated sender: d@ilvokhin.com) by mail.ilvokhin.com (Postfix) with ESMTPSA id BD8E3D0896; Wed, 20 May 2026 11:35:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ilvokhin.com; s=mail; t=1779276943; bh=fxBqUM9QMhCc7/xYGwYeGP+hV4g1ifsVOKoXgGqJ0+Q=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=XKbT3kpXC4RpOmMxnNqIM5QRxgCHU6DbI2HO8Ji1VE3z9TnHV3D58cQ3adwgXWs6R o48tDbnnwquJY9KBjkkboClFLT5riqL5Y9a/J6B4/6Volmqn1XfJkJ+gubPdvYQEgh C8g7IDD4iXyijMe6SRgV5fXnAbweo+AgwfIr6kXU= Date: Wed, 20 May 2026 11:35:40 +0000 From: Dmitry Ilvokhin To: Johannes Weiner Cc: Andrew Morton , Vlastimil Babka , Suren Baghdasaryan , Michal Hocko , Brendan Jackman , Zi Yan , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH] mm/page_alloc: fix defrag_mode for non-reclaimable allocations Message-ID: References: <20260518163736.173910-1-d@ilvokhin.com> <20260518132422.8cfec729a4d7e974c87ace72@linux-foundation.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, May 19, 2026 at 11:28:39AM -0400, Johannes Weiner wrote: > On Tue, May 19, 2026 at 01:47:52PM +0000, Dmitry Ilvokhin wrote: > > On Mon, May 18, 2026 at 01:24:22PM -0700, Andrew Morton wrote: > > > On Mon, 18 May 2026 16:37:36 +0000 Dmitry Ilvokhin wrote: > > > > > > > When defrag_mode is enabled, ALLOC_NOFRAGMENT is enforced to prevent > > > > migratetype fallbacks and keep pageblocks clean. The allocator relies on > > > > reclaim and compaction to free pages of the correct type before allowing > > > > fallback as a last resort. > > > > > > > > However, non-reclaimable allocations such as GFP_ATOMIC cannot invoke > > > > direct reclaim or compaction. With defrag_mode=1, these allocations hit > > > > the !can_direct_reclaim bailout in __alloc_pages_slowpath() with > > > > ALLOC_NOFRAGMENT still set, and fail without ever attempting a fallback. > > > > > > > > This causes a large number of SLUB allocation failures for > > > > skbuff_head_cache under network-heavy workloads, despite free memory > > > > being available in other migratetype freelists. > > > > > > > > Clear ALLOC_NOFRAGMENT and retry before giving up on allocations that > > > > cannot reclaim, following the same pattern used after reclaim/compaction > > > > exhaustion later in the slowpath. > > > > > > Thanks. Sashiko asked a couple of things: > > > > > > https://sashiko.dev/#/patchset/20260518163736.173910-1-d@ilvokhin.com > > > > > > I'm not sure what to make of the first one - we aren't holding any locks > > > in there which prevent concurrent cpuset or zonelist alterations > > > anyway (?). > > > > > > But your change might violate the later comment `No "goto retry;" can be > > > placed above this check * unless it can execute just once'? > > > > Thanks for taking a look, Andrew. > > > > Goto retry can execute at most once, since ALLOC_NOFRAGMENT is cleared > > before the jump, so on the next iteration the condition is false and we > > fall through to goto nopage. This is the similar to the existing > > can_retry_reserves path. > > Yes, it's just a one-off retry with relaxed fragmentation rules, no > need to re-evaluate the cpuset. So this looks fine to me. > > > Just for the sake of keeping everything in one place. Another point > > Sashiko raised. > > > > "Will allocations hitting this PF_MEMALLOC check, or the __GFP_NORETRY check > > further down in the function, still fail prematurely under defrag_mode=1? > > Because these terminal error paths also jump directly to the nopage label, > > they skip the normal ALLOC_NOFRAGMENT clearing at the bottom of the slowpath. > > Should we also clear ALLOC_NOFRAGMENT and retry for these paths so they are > > allowed to fall back rather than failing outright?" > > > > I think by the time we reach the PF_MEMALLOC check, ALLOC_NOFRAGMENT has > > already been cleared, since we set only ALLOC_NO_WATERMARKS and > > ALLOC_KSWAPD in reserve_flags, when PF_MEMALLOC is set. > > Yes, that's correct. alloc_flags gets overwritten, losing NOFRAGMENT, > for privileged requests. And then we retry with that already. > > > For GFP_NORETRY, we can do direct reclaim (compared to GFP_ATOMIC case), > > so we either succeed or not, we don't need another round. > > This is an interesting question. > > GFP_NORETRY can reclaim and compact once, yes, but ALLOC_NOFRAGMENT is > still a higher bar, increasing the likelihood of failure. > > However, unlike GFP_ATOMIC, GFP_NORETRY are usually speculative > allocations with reasonable fallback options (like slub's optimistic > higher order requests). > > The idea behind defrag_mode is to not fragment until the alternative > is OOM. For GFP_ATOMIC, failing is an OOM-like event. For the other > nopage cases, it's more about "my favorite thing isn't available". > > So I'd say let's fix GFP_ATOMIC and leave the other cases alone unless > somebody specifically brings it up as an issue. > > However, there is one catch: GFP_ATOMIC is not its own flag. You're > gating on can_direct_reclaim which is also true for optimistic things > like mTHP allocations (GFP_TRANSHUGE_LIGHT e.g.). We don't want to > fragment for those, either. > > So I think you'd want to check at least if __GFP_KSWAPD_RECLAIM is set > (which it is for GFP_ATOMIC) to decide between fragmenting and > failing. If the caller doesn't even set that, it's a good indicator > that they're purely speculative, and failing is the better option. Thanks for taking a look, Johannes. Makes sense to me. I'll add a check for __GFP_KSWAPD_RECLAIM. > > With that, > > Acked-by: Johannes Weiner