From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 903923ACF17 for ; Mon, 27 Jul 2026 10:52:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785149551; cv=none; b=gSVmBTz38E/4sWlEXwHlxi6V33YoyxuWZksW263ziFTLjCkEFyDxk8kIO8HIiCWA6xRDnLQ7J038I6pAeKPAfFVPCgrUbkHIh8LUGTyWaaMzKIQFRPVBkUEmjJ4c8QIYZ7RBaKaRv02dAASn5+ECeSZo3OlIy92O59/Qra+Q1gE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785149551; c=relaxed/simple; bh=KoZ3PXnlSoijQQJSC7cm9mjop43zEA79PCBnSaPQlqA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=uL0mLWJFcfYYLZ3Ubk0Q4sYrSPem2a5mXa+qSE/+FeORpd7w9S4gxTH8/oS1lY4Xd+hqaRjHN37Tj3srP1vdXEv/BGbaYWxblvDSvrAMUJ8cAuvDLzu8nV3d/d/gpoAWtvieESRth6VKLVyjI73mRMfu0lIvwvXxTTDcy8bP+Qo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=RRSWqDvT; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="RRSWqDvT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3A28B1F000E9; Mon, 27 Jul 2026 10:52:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785149549; bh=00LIXUvOaLfOfFLR8KL4kXbH6LSbYZj9ScEAROEus0A=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=RRSWqDvTqkJFaqCYCzXXgsnImK8fgb168F113cn4oGO5EY70fLN5d1DXcYoE+5hPz 1FN2odxd8bM9nLyeFkq4POOehPIdB96uq7WrDATPVReAyO8MhpKh7NnYrSjdFUttYK 4jkgX8zw/YqkE07LLZJKW7hcsrb5mHobtvzadECjvCt0Nf211ipfODsJGG1al3baUN wiaGjIrscwYEgc2mn5FCAh/jMSjOJ8HkKx0qorNIyfLJsVFq3WULnuF0N0yXmXbfaH +JhiAWz9gR7LkFkFNwFG/aQvMRZ9VjI1fusrUvvSBlNsnkWVf+JBf1to9txlIBEcXK +muWrd4qyARyw== Date: Mon, 27 Jul 2026 11:52:11 +0100 From: "Lorenzo Stoakes (ARM)" To: Zi Yan Cc: Nimrod Oren , Andrew Morton , David Hildenbrand , Baolin Wang , "Liam R. Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Liu Song , Hao Peng , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH] mm/khugepaged: cap pageblock contribution to min_free_kbytes Message-ID: References: <20260716173504.760369-1-noren@nvidia.com> <8E9B69B5-0FBA-4CE5-B737-B83AD1FE52C3@nvidia.com> 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: <8E9B69B5-0FBA-4CE5-B737-B83AD1FE52C3@nvidia.com> On Fri, Jul 17, 2026 at 10:58:45AM -0400, Zi Yan wrote: > On 17 Jul 2026, at 9:36, Lorenzo Stoakes (ARM) wrote: > > > On Thu, Jul 16, 2026 at 08:35:04PM +0300, Nimrod Oren wrote: > >> When THP is enabled, khugepaged can raise min_free_kbytes through > >> set_recommended_min_free_kbytes(), using a heuristic that scales with > > > > Nitty, but that makes it sound like khugepaged (the process) itself comes in and > > changes it, rather set_recommended_min_free_kbytes() sets it higher if > > hugepage_enabled(). > > > >> pageblock_nr_pages. With MIGRATE_PCPTYPES equal to 3, the formula > >> accounts for 11 pageblocks for every eligible populated zone before > >> capping the result at 5% of low memory. > >> > >> This is reasonable when a pageblock is 2MB, as on common 4KB page > >> configurations, but scales poorly with larger base page sizes. With > >> the default arm64 pageblock sizes, the contribution per eligible zone > >> before the 5% cap is: > >> > >> 4KB pages: 2MB pageblock, 22MB per zone > >> 16KB pages: 32MB pageblock, 352MB per zone > >> 64KB pages: 512MB pageblock, 5.5GB per zone > >> > >> The 5% limit does not make this harmless. On an arm64 system with > >> 128GB of RAM and 64KB pages, the current calculation resulted in > >> roughly 6.3GB for min_free_kbytes. The corresponding aggregate min, low > >> and high watermarks were 6.3GB, 7.8GB and 9.4GB. > > > > Yeah this is an ongoing issue (as you note below). We definitely need to find a > > better solution. > > > >> > >> Automatically maintaining multiple GB of additional free memory to > >> improve the probability of 512MB THP allocations seems disproportionate > >> for an opportunistic background optimization. > > > > The fact it's in khugepaged.c doesn't mean it's just about khugepaged, it's > > about avoiding fragmentation for the purposes of obtaining THP pages which can > > also be obtained from page fault and MADV_COLLAPSE. > > > >> > >> This RFC proposes capping the pageblock contribution to the > >> recommendation at 2MB. This would limit the contribution to 22MB per > >> eligible zone while leaving configurations with pageblocks of 2MB or > >> smaller unchanged. > > > > I guess you're trying to do the most conservative thing possible - but it's a > > bit of a cheat really. > > > > I mean does it really make sense to reserve 2 MB for the purposes of obtaining > > 512 MB PMDs? It's essentially giving up on the idea of getting PMD THP > > altogether but pretending you are still doing so. > > Nimrod and I had a discussion about this. It seems that on 64KB base page > systems, 2MB mTHP is more reasonable than 512MB. I also assumed that people > might use 2MB mTHP instead. min_free_kbytes targets getting PMD THP when > under memory pressure, otherwise PMD THP still can be obtained. Yeah, it seems 512 MiB PMDs are really a special case. Though Usama mentioned that he found that sizing to be useful on larger machines I remember from LSF! > > But I also now realize that capping pageblock size at 2MB might also hurt > other >4KB base page systems, like 16KB base pages, since its per zone > reservation is 352MB, much bigger than 22MB. Right. I really don't think changing the pageblock is the right way forward, without significant rework of how the pageblock stuff works. I really dislike 'pretend page blocks are size X' and I also really dislike 'cap page block size without changing default assumptions about PMDs'. The architecture where this matters meaningfully (arm64) is one where mTHP is available so I think long-term a reworked defragmentation model that accounts for mTHP as well as PMD-sized THP is the way forward. (I also think it's a little crazy we go to lengths to defragment just for PMD khugepaged). > > > > > I wonder if it wouldn't just be better to explicitly check the page table > > size and to not even bother trying for > 4 KiB with a comment explaining > > that it's not a worthwhile trade-off. > > > > I think that'd be the more honest version of this patch wouldn't it? > > > > Or maybe we need not be so concerneda bout scaling across zones... > > > >> > >> Earlier proposals to resolve this problem involved disabling default > >> THP on affected systems [1], adding a knob to disable the > >> recommendation [2], or deriving the recommendation from the largest > >> enabled THP order [3]. A fixed cap requires no new knob and avoids > >> large changes to global watermarks when THP policy is changed at > >> runtime. > > > > How often are people turning THP completely off or on again though? > > Not only just turning THP on or off, but also enabling a new mTHP size > or disabling an existing mTHP size. (e.g., anon_enabled_store() used > by all mTHP anon_ctrl_attr_grp). Right yeah, but my point is that set_recommended_min_free_kbytes() only gives a different result if hugepage_enabled() is different. > > > > >> > >> Reducing PAGE_BLOCK_MAX_ORDER remains possible, but it is a build-time > >> policy that changes pageblock granularity globally. Its effects extend > >> beyond the scope of adjusting min_free_kbytes. > >> > >> [1] https://lore.kernel.org/all/CAPm50aLPxJCiVTqqwiz00oMNiqHggB84sXB3x=tv_HUAd5UktQ@mail.gmail.com/ > >> [2] https://lore.kernel.org/all/20230817035155.84230-1-liusong@linux.alibaba.com/ > >> [3] https://lore.kernel.org/all/20250606143700.3256414-1-usamaarif642@gmail.com/ > >> > >> Signed-off-by: Nimrod Oren > > > > Thanks in any case for taking a more pragmatic approach here. > > > >> --- > >> mm/khugepaged.c | 17 +++++++++++++---- > >> 1 file changed, 13 insertions(+), 4 deletions(-) > >> > >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c > >> index 27e8f3077e80..c0570bbb1f4e 100644 > >> --- a/mm/khugepaged.c > >> +++ b/mm/khugepaged.c > >> @@ -23,6 +23,7 @@ > >> #include > >> #include > >> #include > >> +#include > >> > >> #include > >> #include "internal.h" > >> @@ -3066,6 +3067,7 @@ void set_recommended_min_free_kbytes(void) > >> { > >> struct zone *zone; > >> int nr_zones = 0; > >> + unsigned long capped_pageblock_nr_pages; > >> unsigned long recommended_min; > >> > >> if (!hugepage_enabled()) { > >> @@ -3084,16 +3086,23 @@ void set_recommended_min_free_kbytes(void) > >> nr_zones++; > >> } > >> > >> - /* Ensure 2 pageblocks are free to assist fragmentation avoidance */ > >> - recommended_min = pageblock_nr_pages * nr_zones * 2; > >> + /* > >> + * Limit the pageblock contribution to 2 MiB. Larger pageblocks can > >> + * otherwise make min_free_kbytes disproportionately large. > >> + */ > >> + capped_pageblock_nr_pages = > >> + min(pageblock_nr_pages, SZ_2M / PAGE_SIZE); > > > > Yeah this just feels like a hack, to be honest "let's pretend a page block > > is 2 MiB" essentially :) > > > >> + > >> + /* Ensure 2 capped units are free to assist fragmentation avoidance */ > >> + recommended_min = capped_pageblock_nr_pages * nr_zones * 2; > >> > >> /* > >> - * Make sure that on average at least two pageblocks are almost free > >> + * Make sure that on average at least two capped units are almost free > >> * of another type, one for a migratetype to fall back to and a > >> * second to avoid subsequent fallbacks of other types There are 3 > >> * MIGRATE_TYPES we care about. > >> */ > >> - recommended_min += pageblock_nr_pages * nr_zones * > >> + recommended_min += capped_pageblock_nr_pages * nr_zones * > >> MIGRATE_PCPTYPES * MIGRATE_PCPTYPES; > > > > we're really getting into faking stuff out realms here - pretending a page > > block is 2 MiB then doing calculations on it as if they were. > > > > We'd be better off (and more honest) with something like: > > > > - if (!hugepage_enabled()) { > > + /* Avoid excessive memory reservation if page blocks are very large. */ > > + if (!hugepage_enabled() || PAGE_SIZE > SZ_4K) { > > > > But that seems perhaps unwise. > > > > Maybe a simpler version would be to just attack the issue we actually have > > with it - it uses too much damn memory :) > > > > So maybe something like the attached? (you'd probably want to make this a > > sysctl or at least a defined value of course) > > The approach below makes sense to me. In terms of the actual cap size, > I asked Codex to check what possible pageblock_size and their reserved > sizes: > > 1. 1MB pageblock size (s390) and 11MB per zone, > 2. 2MB pageblock size and 22MB per zone, > 3. 8MB pageblock size and 88MB per zone, > 4. 16MB pageblock size and 176MB per zone, > 5. 32MB pageblock size and 352MB per zone, > 6. 128MB pageblock size and 1.375GB per zone, > 7. 512MB pageblock size and 5.5GB per zone. > > 1GB makes sense to me. 512MB might also work. Yeah I felt this was a bit of a 'dumb' solution but ultimately if we are saying 'restrict how much memory this uses to something sane' then that's what we should just do. It might be a good bandaid at least for this issue which is ongoing, well known, and painful. I think at 352MB per zone and 2-3 zones for 32 MiB pageblock, plus reserves of 2*512 MiB for 512 MiB pageblock makes 1 GiB a sensible choice here overall. > > > > > > General idea is - if we're going to cap, let's just cap instead of > > pretending that page blocks are of a different size. > > > > Sounds good to me. Thank you for the suggestion. Thanks! > > >> > >> /* don't ever allow to reserve more than 5% of the lowmem */ > >> -- > >> 2.45.0 > >> > > > > Cheers, Lorenzo > > > > ----8<---- > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > index 617bca76db49..d9e1e7de5494 100644 > > --- a/mm/khugepaged.c > > +++ b/mm/khugepaged.c > > @@ -3092,9 +3092,13 @@ void set_recommended_min_free_kbytes(void) > > recommended_min += pageblock_nr_pages * nr_zones * > > MIGRATE_PCPTYPES * MIGRATE_PCPTYPES; > > > > - /* don't ever allow to reserve more than 5% of the lowmem */ > > + /* > > + * don't ever allow the reservation of more than 1 GiB or 5% of lowmem > > + * (whichever is smaller). > > + */ > > recommended_min = min(recommended_min, > > (unsigned long) nr_free_buffer_pages() / 20); > > + recommended_min = min(recommended_min, SZ_1G); > > recommended_min <<= (PAGE_SHIFT-10); > > > > if (recommended_min > min_free_kbytes) { > > > Best Regards, > Yan, Zi Cheers, Lorenzo