From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-112.freemail.mail.aliyun.com (out30-112.freemail.mail.aliyun.com [115.124.30.112]) (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 5638248A8DB for ; Tue, 15 Sep 2026 11:09:19 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.112 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789470563; cv=none; b=IjC2uqfQ9Yye7zBfez4q9VXsmfzCVWvv4eoL+y80QTDcItqXfFwEtX+KB5W1zKY0qowIoGGdsWNcVWxRjTnsv2kEn6QEap6krUJPIaDY7AFzc+zqyNvq2PTu9x1ai8S+hze3pgIGNjbPh66Qa3qn0BWWMW3tbuedi84QZ2eaRWo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789470563; c=relaxed/simple; bh=a2NtyJCmSqKMC7hqNOlmB1JTEbjRuG+ZiRUwUk/PIzs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=EcVUDZHzrE354vxS0hmudGK9JHa9jd0NyknRue9jp+74fYMGzrhop/fkDEwJisWBPLpTiywzBPE0iymLOozNkIZnaEvVcBxzf0YOde0MR6PTPqQtuOi36I+DwIKwfh24FHE2R/E+NJNgxxjrlt6Zfzw1Ogb9qGr/NRCJY5mK6Vs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=rouD+5vL; arc=none smtp.client-ip=115.124.30.112 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="rouD+5vL" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1789470550; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=vxNn5ZYVwKKzqCEAyb+0n26iLMvhwicQTy1pDi/GIeI=; b=rouD+5vLFNEMcnYJ1uC5j0Q720OAenuIewIKfGXDqxR7/HnHoZnLXNSDgySDT8ljpOS2PewPmTNq8sOZaer/hNVzP+eaIldwVb7eLVHVTRH5IrziHDPNDcbzpeDFeOtKL1OxaDPb0ayqzY8wGs3EnrEfj195MHkJV4Fx7eX1l5Q= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R101e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033045133197;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=18;SR=0;TI=SMTPD_---0XB1OONN_1789470547; Received: from 30.74.144.126(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XB1OONN_1789470547 cluster:ay36) by smtp.aliyun-inc.com; Tue, 15 Sep 2026 19:09:08 +0800 Message-ID: <1e91c48c-cfdf-4a40-bf1c-88d8e0295c5a@linux.alibaba.com> Date: Tue, 15 Sep 2026 19:09:06 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 08/12] mm/collapse: separate scanning a PTE table from collapsing it To: Kiryl Shutsemau , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , Zi Yan Cc: "Kiryl Shutsemau (Meta)" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, "Liam R . Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Usama Arif , Vlastimil Babka , Jann Horn References: <20260910120238.2529819-1-kirill@shutemov.name> <20260910120238.2529819-9-kirill@shutemov.name> From: Baolin Wang In-Reply-To: <20260910120238.2529819-9-kirill@shutemov.name> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 9/10/26 8:02 PM, Kiryl Shutsemau wrote: > From: "Kiryl Shutsemau (Meta)" > > A collapse is two jobs. One reads a PTE table under mmap_lock and decides > whether the range is worth collapsing. The other allocates, isolates, > copies and flushes, and wants the lock given up first. > > collapse_single_pmd() did both, so the boundary between them was somewhere > in the middle of a function. > > Give each half its own function: > > - collapse_scan_pmd() scans one table and only reads. The anonymous > scan that used to carry that name keeps its body as > collapse_scan_anon_pmd(), and collapse_scan_pmd() is now the entry > that picks the anonymous or the file side. > > - collapse_run_pmd() does the collapse the scan asked for. > SCAN_SUCCEED from the scan means there is something to run; anything > else is why there is not. > > collapse_single_pmd() is now the two of them with the mmap_lock drop in > between, so its callers see what they saw before. > > What the scan found and the run needs travels in collapse_control. For > an anonymous table that is the orders and the referenced and swapped-out > counts. For a file it is the file itself, the offset in it, and whether > the PMD folio is already in the page cache. > > The file side moves with the anonymous one. collapse_scan_file() used to > run with mmap_lock already given up, and called collapse_file() itself > when the page cache looked worth it. It now runs under the lock like the > anonymous scan and only judges; the run does the collapse. A file > collapse works on the page cache and never sees a VMA, so the scan takes > the file reference while it still has one and the run gives it back. > > That changes what a refused file table costs khugepaged. Every file > table it scanned used to end its pass over that mm, because the lock had > been dropped to scan it; now only a table it goes on to collapse does. > > Two things on the file side stop being rescanned. When the page cache > already holds the PMD folio, the scan says so and the run goes straight > to retracting the PTE table. A run that refuses dirty pages and may > write them back retries collapse_file() alone. The checks the scan makes > ahead of it are ones collapse_file() repeats under the page cache lock. > > Tracing changes with it. mm_khugepaged_scan_pmd and > mm_khugepaged_scan_file used to fire after the collapse, so for an > accepted table their status field carried what the collapse made of it. > They now fire before it and read SCAN_SUCCEED for an accepted table. What > the collapse then made of it is for mm_collapse_huge_page and > mm_khugepaged_collapse_file to report. > > Assisted-by: LLM > Signed-off-by: Kiryl Shutsemau (Meta) > --- > mm/collapse.h | 16 ++++++ > mm/khugepaged.c | 147 ++++++++++++++++++++++++++++++++++++------------ > 2 files changed, 128 insertions(+), 35 deletions(-) > > diff --git a/mm/collapse.h b/mm/collapse.h > index 7044dc71c7c2..346859a2184f 100644 > --- a/mm/collapse.h > +++ b/mm/collapse.h > @@ -88,6 +88,22 @@ struct collapse_control { > > /* Each bit marks a PTE the scan accepted as a collapse source */ > DECLARE_BITMAP(eligible_ptes, MAX_PTRS_PER_PTE); > + > + /* > + * What a scan found and the run after it needs. Live only between the > + * two, and read by nobody else. > + * > + * The file side takes a reference while it still has the VMA, since a > + * file collapse works on the page cache and never sees one; the run is > + * what gives it back. A scan that found the PMD folio already in the > + * cache leaves only the PTE table to retract. > + */ > + unsigned long scan_orders; > + int scan_referenced; > + int scan_unmapped; > + struct file *scan_file; > + pgoff_t scan_pgoff; > + bool scan_retract_only; > }; [snip] > + > +static enum scan_result collapse_scan_pmd(struct vm_area_struct *vma, > + unsigned long addr, struct collapse_control *cc) > { > - struct mm_struct *mm = vma->vm_mm; > - bool triggered_wb = false; > enum scan_result result; > - struct file *file; > pgoff_t pgoff; > > - mmap_assert_locked(mm); > + mmap_assert_locked(vma->vm_mm); > + /* Whatever the last scan found has to have been run by now */ > + if (WARN_ON_ONCE(cc->scan_file)) { > + fput(cc->scan_file); > + cc->scan_file = NULL; > + } > > if (vma_is_anonymous(vma)) > - return collapse_scan_pmd(mm, vma, addr, lock_dropped, cc); > + return collapse_scan_anon_pmd(vma, addr, cc); > > - file = get_file(vma->vm_file); > pgoff = linear_page_index(vma, addr); > + result = collapse_scan_file(vma->vm_mm, addr, vma->vm_file, pgoff, cc); > + switch (result) { > + case SCAN_SUCCEED: > + cc->scan_retract_only = false; > + break; > + case SCAN_PTE_MAPPED_HUGEPAGE: > + /* > + * The page cache already holds the PMD folio; what is left is > + * to retract the PTE table, which is the run's job. > + */ > + cc->scan_retract_only = true; > + result = SCAN_SUCCEED; > + break; > + default: > + return result; > + } > > - mmap_read_unlock(mm); > - *lock_dropped = true; > + /* > + * A file collapse works on the page cache and never sees a VMA, so take > + * what it needs from this one while it is still here. > + */ > + cc->scan_file = get_file(vma->vm_file); > + cc->scan_pgoff = pgoff; > + return result; > +} > + > +static enum scan_result collapse_run_pmd(struct mm_struct *mm, > + unsigned long addr, struct collapse_control *cc) > +{ > + struct file *file = cc->scan_file; > + bool triggered_wb = false; > + enum scan_result result; > + pgoff_t pgoff; > + > + if (!file) > + return mthp_collapse(mm, addr, cc->scan_referenced, > + cc->scan_unmapped, cc, cc->scan_orders); You already record scan_referenced, scan_unmapped, and scan_orders in 'cc'. It looks like simply passing 'cc' as a parameter should be enough.