From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 057C9481229; Tue, 12 May 2026 07:42:49 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778571770; cv=none; b=OKuKBTo/jaF6iJ2obzN/j/bFSOIPZ5KD8MZtZo4SwSEFRSanIDvCx92M7guu/h4tqdeRUPlJCO/umKA89wzA4C4vvwVKR9+dMEXkyfaQn/PJnpwP23c/jq9r5PXv6OYA9KDV7YM7WqX9UsP6N/QqV8seR0bBGTIjxRq4UKXCvNQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778571770; c=relaxed/simple; bh=iB8tMCv0MnNsT4JdZSVwpaqxrQgKB7oJIZ1Ch8nbSQE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=WejS8785m3dTMV3Ec5LbROsowP/xbcbX2LxOXMwYwDl9rc/uyDy9wJyaq12Vq+zO1dcTIJm/QiuB+UwF0nMfI+PImcHVfD4D1BXF6p6C8btB1yAXptN4shJfsusTB96GBENc2+wxC7UoYaGFvlyDjj43QLeP6FOSr8Kbu8yfqfQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=AXMgw/kW; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="AXMgw/kW" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 567D9C2BCB0; Tue, 12 May 2026 07:42:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778571769; bh=iB8tMCv0MnNsT4JdZSVwpaqxrQgKB7oJIZ1Ch8nbSQE=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=AXMgw/kWeXATu0nbAK7QZZ8AuBWGNIn0QfGbj0HLOhiUj+fUEZM6qPW+6j0WW+Rh3 mJOswvrr21Mk22eRCPWcEmuOtDN9w7/rWEX/cgJ2Z22jGV0Mq3CQNLzjGfzJ5qhqOl Jd+8UCodmDz5uxv/1M9vXs1W4CTu8q0Y2TvZqEEiTAbm3U6DvYhb4S8mdMI03+GSpx HxSvUoofnBHiZHIfwcgWvMqCA+LC3fRWXTQkPV4J1MyCai8N2ds+e+glhrAQp/N2L+ 2PM5LXPQbTEAPg+cQhNe2zVGPj6lMPMkLuEYE8pf3IewN5wc0p3ruMZHCvXJmDZq/c tYsmkI+ofRRMg== Message-ID: Date: Tue, 12 May 2026 09:42:37 +0200 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v17 05/14] mm/khugepaged: require collapse_huge_page to enter/exit with the lock dropped To: Nico Pache , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org Cc: aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, lance.yang@linux.dev, liam@infradead.org, ljs@kernel.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com References: <20260511185817.686831-1-npache@redhat.com> <20260511185817.686831-6-npache@redhat.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/11/26 20:58, Nico Pache wrote: > Currently the collapse_huge_page function requires the mmap_read_lock to > enter with it held, and exit with it dropped. This function moves the > unlock into its parent caller, and changes this semantic to requiring it > to enter/exit with it always unlocked. > > In future patches, we need this expectation, as for in mTHP collapse, we > may have already have dropped the lock, and do not want to conditionally > check for this by passing through the lock_dropped variable. > > No functional change is expected as one of the first things the > collapse_huge_page function does is drop this lock before allocating the > hugepage. > > Signed-off-by: Nico Pache > --- > mm/khugepaged.c | 18 ++++++++++-------- > 1 file changed, 10 insertions(+), 8 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 27465161fa6d..37a5f6791816 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1199,6 +1199,14 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru > return SCAN_SUCCEED; > } > > +/* > + * collapse_huge_page expects the mmap_read_lock to be dropped before > + * entering this function. "before entering." Talking about "this function" after naming it sounds odd. Also, there is only an "mmap_lock". > The function will also always return with the lock > + * dropped. "collapse_huge_page expects the mmap_lock to be unlocked before entering and will always return with the lock unlocked." Or something simple like that? > The function starts by allocation a folio, which can potentially > + * take a long time if it involves sync compaction, and we do not need to hold > + * the mmap_lock during that. We must recheck the vma after taking it again in > + * write mode. > + */ "... to avoid holding the mmap_lock while allocating a THP, as that could trigger direct reclaim/compaction. Note that the VMA must be rechecked after grabbing the mmap_lock again." ? Ending up with something like "collapse_huge_page expects the mmap_lock to be unlocked before entering and will always return with the lock unlocked, to avoid holding the mmap_lock while allocating a THP, as that could trigger direct reclaim/compaction. Note that the VMA must be rechecked after grabbing the mmap_lock again." > static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long address, > int referenced, int unmapped, struct collapse_control *cc) > { > @@ -1214,14 +1222,6 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long a > > VM_BUG_ON(address & ~HPAGE_PMD_MASK); > > - /* > - * Before allocating the hugepage, release the mmap_lock read lock. > - * The allocation can take potentially a long time if it involves > - * sync compaction, and we do not need to hold the mmap_lock during > - * that. We will recheck the vma after taking it again in write mode. > - */ > - mmap_read_unlock(mm); > - > result = alloc_charge_folio(&folio, mm, cc, HPAGE_PMD_ORDER); > if (result != SCAN_SUCCEED) > goto out_nolock; > @@ -1526,6 +1526,8 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm, > out_unmap: > pte_unmap_unlock(pte, ptl); > if (result == SCAN_SUCCEED) { > + /* collapse_huge_page expects the lock to be dropped before calling */ > + mmap_read_unlock(mm); > result = collapse_huge_page(mm, start_addr, referenced, > unmapped, cc); > /* collapse_huge_page will return with the mmap_lock released */ Acked-by: David Hildenbrand (Arm) -- Cheers, David