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 BD44B2853E9 for ; Mon, 23 Feb 2026 14:41:32 +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=1771857692; cv=none; b=ZzJno7YuKK6ivdta3HKholayroFwfdFBTGMz1E4btTk1ilf1XmQ6H0qHi7+8EtobpccTZkSsbYbaORgGluIRkf7RCXylF5+2tHK2NWyRkDPuAsWKN0/NVQKy9e49ADFmjkHliOJCj9qEq1V/W/Cydq4ySBjVkiVBf6XImUjDizU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771857692; c=relaxed/simple; bh=QD+SNLUSuTOZKVO6kq/rHyuVfqWXDuG6rqionI0X+CE=; h=Message-ID:Date:MIME-Version:Subject:From:To:Cc:References: In-Reply-To:Content-Type; b=o9yCJnopAEG7J5y7wN3CgsF51QDQ854l+NgCCYvN16DNW1MHtGzf77vBH88N1Lk448O81ZH9MX3u4xnmFmsuTDyrGeVRNIWeiOv4H19UvGD/8mw7ABEmrVeAwvL3Gz6GbjI2iJcu89ONP1NTlpBaSb0mY4CcE7JsepAhDzKN0oU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=vRne8mVm; 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="vRne8mVm" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0CA9CC116C6; Mon, 23 Feb 2026 14:41:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1771857692; bh=QD+SNLUSuTOZKVO6kq/rHyuVfqWXDuG6rqionI0X+CE=; h=Date:Subject:From:To:Cc:References:In-Reply-To:From; b=vRne8mVm1wUiMLJe3Q5/Vuut0AOOqPzy4TiDA3vajKEww9dOqSPce0/sidKexz+5d sMkziN7KI1NkCQHG91cG4a0KyhtK+WEGF/BduZ79fojPH85oPjME4/Z35nb6S7JQ7j 0/UnDhWRIHKBAkxHjHmnwzVljRQvHePlmFHGedRJY/TysUX8W5ar5ezNZN6aSHQW95 Tozog1PMIGvWkvtR1X85A9HOuBPeP1lY6jZh74Dl4aDKhrBIu00wuqy3jFNDoM7kOD egkN/L+xlTtVBLhhDDtptkSReTb/F8uJqpJF79HnILbTJLeikuBXByaY5++GyWgCkM OiDlr5yT5hCeQ== Message-ID: <92e9d637-988a-4201-8c8b-c29987b727bb@kernel.org> Date: Mon, 23 Feb 2026 15:41:15 +0100 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 mm-unstable v1 5/5] mm/khugepaged: unify khugepaged and madv_collapse with collapse_single_pmd() From: "David Hildenbrand (Arm)" To: Nico Pache Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, 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, jackmanb@google.com, jack@suse.cz, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, lance.yang@linux.dev, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, 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: <20260212021835.17755-1-npache@redhat.com> <20260212022512.19076-1-npache@redhat.com> <164bfaf0-5e8a-4bd2-a04c-93d61856a941@kernel.org> <62d53608-ed7b-42fe-baad-3023764383a3@kernel.org> Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 2/23/26 15:24, David Hildenbrand (Arm) wrote: > On 2/12/26 21:26, Nico Pache wrote: >> On Thu, Feb 12, 2026 at 1:04 PM David Hildenbrand (Arm) >> wrote: >>> >>> >>> Having triggered_wb set where writeback is not actually triggered is >>> suboptimal. >> >> It took me a second to figure out what you were referring to, but I >> see it now. if we return SCAN_PAGE_D_OR_WB but the can_writeback fails >> it still retries. >> >> Would be an appropriate solution if can_writeback fails to modify the >> return value. >> ie) >> >> if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK) { >>      if (mapping_can_writeback(file->f_mapping)) { >>         const loff_t lstart = (loff_t)pgoff << PAGE_SHIFT; >>         const loff_t lend = lstart + HPAGE_PMD_SIZE - 1; >> >>         filemap_write_and_wait_range(file->f_mapping, lstart, lend); >>      } else { >>         result = SCAN_(SOMETHING?) >>      } >> } >> fput(file); >> >> we dont have a enum that fits this description but we want one that >> will continue. > > I stared at the patch and possible ways to change it, but I wondered > whether this refactoring is really the right approach. > > The whole mmap locking just makes this all very weird. > > Let me think about it some more. > As we don't really need to re-grab the MM lock, I think we can just do: diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 844a8a27845e..d546ff173833 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -2395,6 +2395,7 @@ static enum scan_result collapse_single_pmd(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; @@ -2409,14 +2410,22 @@ static enum scan_result collapse_single_pmd(unsigned long addr, mmap_read_unlock(mm); *mmap_locked = false; + +retry: result = collapse_scan_file(mm, addr, file, pgoff, cc); + /* + * For MADV_COLLAPSE, try to writeback once to retry the collapse + * immediately. + */ if (!cc->is_khugepaged && result == SCAN_PAGE_DIRTY_OR_WRITEBACK && - mapping_can_writeback(file->f_mapping)) { + !triggered_wb && mapping_can_writeback(file->f_mapping)) { const loff_t lstart = (loff_t)pgoff << PAGE_SHIFT; const loff_t lend = lstart + HPAGE_PMD_SIZE - 1; filemap_write_and_wait_range(file->f_mapping, lstart, lend); + triggered_wb = true; + goto retry; } fput(file); @@ -2814,9 +2823,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) { enum scan_result result = SCAN_FAIL; - bool triggered_wb = false; -retry: if (!mmap_locked) { cond_resched(); mmap_read_lock(mm); @@ -2838,11 +2845,6 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, if (!mmap_locked) *lock_dropped = true; - if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb) { - triggered_wb = true; - goto retry; - } - switch (result) { case SCAN_SUCCEED: case SCAN_PMD_MAPPED: -- Cheers, David