From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-171.mta0.migadu.com (out-171.mta0.migadu.com [91.218.175.171]) (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 F24043B0AC4 for ; Tue, 9 Jun 2026 16:28:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.171 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781022514; cv=none; b=Am7dJkz/LfKYf91bf742GLoH2HoPJIHDybbgXYHtl86b54DkMdFN7ZQRmdI8uqu9EfpyYvfVMXfyk+FDR5zrljpnr1co16jOhWfpA9YoHPCjNQuqJkPMCmzbDe1C27mr9QWPO39ydLnpDa2L+XqJfScuLYyc/zFcwEtrtJPRzQc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781022514; c=relaxed/simple; bh=iWIFFPa8rBwVx8zjVcHUwKtueOu9fil6pm6DcFg4W8w=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=l9dK7vyxY8cGeJEtuhBpgHCup1R7fI1i5OoVIRU1rxZH7znFptFRmEacXINocBvfhPOa96cXWUrLBgWKMgXGNy3uWdUdkcJhxUVXYfSYmtUcc3VqdmYKa8LNkM1Op5LJ34q+Orjqhl1lrMEyPxaQ/YDQwPkjR12y5oUvgKiiJtM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=aEhNyBBw; arc=none smtp.client-ip=91.218.175.171 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="aEhNyBBw" Message-ID: <9e548d7d-480a-42cb-8912-80f415cd0bb3@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1781022510; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=os2EOsUBH5Y105INP1cDhq+ANm25U6M+e7FDSnCAoH0=; b=aEhNyBBw4zitxBz/ovKzaEhoFC6QXt0dhiYPjJBkpsozOotVZPug2ybeDar2If9kWeb0Vb AEkQDW9LbzLNrY6Ym/PIw5F6HHgW9wCWdZQHCw3AYqBvGXdk2DGaBro6NJPmy1V/3SAe/F Pfg+8/elwy0xHrKooF+Nxq10uFfqDBk= Date: Wed, 10 Jun 2026 00:28:24 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH mm-unstable 1/1] mm/khugepaged: fix PMD collapse swap PTE accounting Content-Language: en-US To: "David Hildenbrand (Arm)" , akpm@linux-foundation.org, ljs@kernel.org Cc: ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260609120443.71864-1-lance.yang@linux.dev> <7d081256-5b30-4e3c-b948-85ba76ad0e1d@kernel.org> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Lance Yang In-Reply-To: <7d081256-5b30-4e3c-b948-85ba76ad0e1d@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 2026/6/9 21:16, David Hildenbrand (Arm) wrote: > On 6/9/26 14:04, Lance Yang wrote: >> From: Lance Yang >> >> mthp_collapse() uses mthp_present_ptes to decide whether a range has >> enough occupied PTEs to try collapse. Swap PTEs accepted by >> collapse_scan_pmd() are counted in unmapped, but are not represented in >> mthp_present_ptes. >> >> When lower orders are enabled, collapse_scan_pmd() relaxes max_ptes_none >> so the scan can cover the whole PMD and build the bitmap. mthp_collapse() >> then checks the PMD-order candidate using the bitmap. >> >> With max_ptes_none set to 0, a range with 511 present PTEs and one swap >> PTE no longer reaches collapse_huge_page(), even though PMD collapse can >> handle swap PTEs up to max_ptes_swap. >> >> Account unmapped PTEs only for PMD order. PMD collapse supports swap PTEs >> through max_ptes_swap, while lower-order mTHP collapse does not currently >> support non-present PTEs. Keep non-present PTEs out of the lower-order >> eligibility check. >> >> Signed-off-by: Lance Yang >> --- >> Sent separately, as discussed in [1], to spell out the PMD-order swap PTE >> case. Patch [2] is still only in mm-unstable, so no Fixes: tag. >> >> [1] https://lore.kernel.org/linux-mm/CAA1CXcD7WAiA1b9GTLAuNZ+kHaFx0SzZwpBkqAZ=s+RHsTUaow@mail.gmail.com/ >> [2] https://lore.kernel.org/linux-mm/20260605161422.213817-12-npache@redhat.com/ >> >> mm/khugepaged.c | 8 ++++++++ >> 1 file changed, 8 insertions(+) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index b12187709f6d..617bca76db49 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -1508,6 +1508,14 @@ static enum scan_result mthp_collapse(struct mm_struct *mm, >> nr_occupied_ptes = bitmap_weight_from(cc->mthp_present_ptes, offset, >> offset + nr_ptes); >> >> + /* >> + * Swap PTEs accepted during the scan are counted in @unmapped, >> + * not in the present-PTE bitmap. Account them for the PMD-order >> + * candidate. >> + */ >> + if (is_pmd_order(order)) >> + nr_occupied_ptes += unmapped; >> + > > LGTM, there is a bit of opportunity for cleanup in the future :) Yes, follow-up cleanup material :) > Acked-by: David Hildenbrand (Arm) Thanks! > For example, as we no longer have the VMA here, collapse_max_ptes_none is > imprecise in uffd VMAs. We might try collapsing where there sure is nothing to > collapse. Oh, good catch. We may end up trying a collapse that cannot really go anywhere ... One for a follow-up. > We could likely handle the userfaultfd_armed() part easier: some indication that > we must not have any pte_none() would be sufficient. Right. By the time we get to mthp_collapse(), we probably only need to carry that as a small "no pte_none" constraint for the candidate range. > Also, I don't see a good reason why uffd would not be allowed to collapse with > zeropages ... it's really just about missing faults due to pte_none(). Makes sense to me. I'll take a look when I get a chance. And yeah, as Lorenzo said, better to clean up the khugepaged mess first before piling more on top :) Cheers, Lance