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 CC12E352FB2; Wed, 14 Jan 2026 20:14:15 +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=1768421655; cv=none; b=bzV/2G8bdbR9OdiQh0YTXyNfZSKkfwuR+xEy8dd3ZrSrUxTKHW1uOBJAN8Zsm7Y/u8kaSbefKWaxpS4iXARoCosFxMBUzmXoNtIE+XoOhuEGVwplMjOIBqr5t1lm8mRw9yy7YWX8bgEveupMFevbcTLJqMjapzbOaqT5y1/sZNU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768421655; c=relaxed/simple; bh=ln4PO0exRBfvEWi16/ut6iLsNheqOyuMABH4qKzzpYw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=IeQzoeI+vdeuSX0znrNLCCdHJRxl8ltNwdJeq1eVo6WUjQsRCct15D4wLC6wz8kRMs8FbY9g7+usT6YSpXvO9CqKRpYOXez/Va2erpgvcyPv0bcfINpb5CWCLMbNav7OnoT+4z5WUpuS0sE/SClVLgy1PIL1YE+75IJmok8I/x4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PXIK6kEv; 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="PXIK6kEv" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5610CC4CEF7; Wed, 14 Jan 2026 20:14:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1768421655; bh=ln4PO0exRBfvEWi16/ut6iLsNheqOyuMABH4qKzzpYw=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=PXIK6kEvsZ23eqAxs3owI5uGeoC7MTSnqlL5xkVO4usRMOdEZdWFedia1iUOyo19R uoZwi4skY9hQvPOwj1AwVz19JM1eAxUlWUKbKBTWoNQweZFseAvzP26ARB7scjXgsr G3n4WW9zvnC0jSkfnNRI25sSPJ6J0taDOJ7iu4Kkx9zI5AM4ObLC/t+4S94Gm/+3ZH ryEz7Tl8/NPYikFnadjg6Fanrt7LVEjM+BDsXkW00BGLcpGIFEdaO7w+TMtnxX1eOd wgqx5cltZN21CPrkk2rgv1FVzx30KGO74TvwQj9X/101XtsLTw3V8djgIvSZV4prJe zq4bDJmlYxtZw== Message-ID: <976561d0-0edb-4f09-8ed1-ab8f85c9aa87@kernel.org> Date: Wed, 14 Jan 2026 21:14:08 +0100 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 V4 2/2] mm/khugepaged: retry with sync writeback for MADV_COLLAPSE To: "Garg, Shivank" , Andrew Morton , Lorenzo Stoakes Cc: Zi Yan , Baolin Wang , "Liam R . Howlett" , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Steven Rostedt , Masami Hiramatsu , Mathieu Desnoyers , Zach O'Keefe , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Branden Moore References: <20251215084615.5283-3-shivankg@amd.com> <20251215084615.5283-7-shivankg@amd.com> <3cc27720-8f53-4b6f-9202-42b3b73928b8@kernel.org> <7a42515f-ae57-4f4d-831c-87689930a797@amd.com> From: "David Hildenbrand (Red Hat)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAa2VybmVsLm9yZz7CwY0EEwEIADcWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCaKYhwAIbAwUJJlgIpAILCQQVCgkIAhYCAh4FAheAAAoJEE3eEPcA/4Naa5EP/3a1 9sgS9m7oiR0uenlj+C6kkIKlpWKRfGH/WvtFaHr/y06TKnWn6cMOZzJQ+8S39GOteyCCGADh 6ceBx1KPf6/AvMktnGETDTqZ0N9roR4/aEPSMt8kHu/GKR3gtPwzfosX2NgqXNmA7ErU4puf zica1DAmTvx44LOYjvBV24JQG99bZ5Bm2gTDjGXV15/X159CpS6Tc2e3KvYfnfRvezD+alhF XIym8OvvGMeo97BCHpX88pHVIfBg2g2JogR6f0PAJtHGYz6M/9YMxyUShJfo0Df1SOMAbU1Q Op0Ij4PlFCC64rovjH38ly0xfRZH37DZs6kP0jOj4QdExdaXcTILKJFIB3wWXWsqLbtJVgjR YhOrPokd6mDA3gAque7481KkpKM4JraOEELg8pF6eRb3KcAwPRekvf/nYVIbOVyT9lXD5mJn IZUY0LwZsFN0YhGhQJ8xronZy0A59faGBMuVnVb3oy2S0fO1y/r53IeUDTF1wCYF+fM5zo14 5L8mE1GsDJ7FNLj5eSDu/qdZIKqzfY0/l0SAUAAt5yYYejKuii4kfTyLDF/j4LyYZD1QzxLC MjQl36IEcmDTMznLf0/JvCHlxTYZsF0OjWWj1ATRMk41/Q+PX07XQlRCRcE13a8neEz3F6we 08oWh2DnC4AXKbP+kuD9ZP6+5+x1H1zEzsFNBFXLn5EBEADn1959INH2cwYJv0tsxf5MUCgh Cj/CA/lc/LMthqQ773gauB9mN+F1rE9cyyXb6jyOGn+GUjMbnq1o121Vm0+neKHUCBtHyseB fDXHA6m4B3mUTWo13nid0e4AM71r0DS8+KYh6zvweLX/LL5kQS9GQeT+QNroXcC1NzWbitts 6TZ+IrPOwT1hfB4WNC+X2n4AzDqp3+ILiVST2DT4VBc11Gz6jijpC/KI5Al8ZDhRwG47LUiu Qmt3yqrmN63V9wzaPhC+xbwIsNZlLUvuRnmBPkTJwwrFRZvwu5GPHNndBjVpAfaSTOfppyKB Tccu2AXJXWAE1Xjh6GOC8mlFjZwLxWFqdPHR1n2aPVgoiTLk34LR/bXO+e0GpzFXT7enwyvF FFyAS0Nk1q/7EChPcbRbhJqEBpRNZemxmg55zC3GLvgLKd5A09MOM2BrMea+l0FUR+PuTenh 2YmnmLRTro6eZ/qYwWkCu8FFIw4pT0OUDMyLgi+GI1aMpVogTZJ70FgV0pUAlpmrzk/bLbRk F3TwgucpyPtcpmQtTkWSgDS50QG9DR/1As3LLLcNkwJBZzBG6PWbvcOyrwMQUF1nl4SSPV0L LH63+BrrHasfJzxKXzqgrW28CTAE2x8qi7e/6M/+XXhrsMYG+uaViM7n2je3qKe7ofum3s4v q7oFCPsOgwARAQABwsF8BBgBCAAmAhsMFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAmic2qsF CSZYCKEACgkQTd4Q9wD/g1oq0xAAsAnw/OmsERdtdwRfAMpC74/++2wh9RvVQ0x8xXvoGJwZ rk0Jmck1ABIM//5sWDo7eDHk1uEcc95pbP9XGU6ZgeiQeh06+0vRYILwDk8Q/y06TrTb1n4n 7FRwyskKU1UWnNW86lvWUJuGPABXjrkfL41RJttSJHF3M1C0u2BnM5VnDuPFQKzhRRktBMK4 GkWBvXlsHFhn8Ev0xvPE/G99RAg9ufNAxyq2lSzbUIwrY918KHlziBKwNyLoPn9kgHD3hRBa Yakz87WKUZd17ZnPMZiXriCWZxwPx7zs6cSAqcfcVucmdPiIlyG1K/HIk2LX63T6oO2Libzz 7/0i4+oIpvpK2X6zZ2cu0k2uNcEYm2xAb+xGmqwnPnHX/ac8lJEyzH3lh+pt2slI4VcPNnz+ vzYeBAS1S+VJc1pcJr3l7PRSQ4bv5sObZvezRdqEFB4tUIfSbDdEBCCvvEMBgoisDB8ceYxO cFAM8nBWrEmNU2vvIGJzjJ/NVYYIY0TgOc5bS9wh6jKHL2+chrfDW5neLJjY2x3snF8q7U9G EIbBfNHDlOV8SyhEjtX0DyKxQKioTYPOHcW9gdV5fhSz5tEv+ipqt4kIgWqBgzK8ePtDTqRM qZq457g1/SXSoSQi4jN+gsneqvlTJdzaEu1bJP0iv6ViVf15+qHuY5iojCz8fa0= In-Reply-To: <7a42515f-ae57-4f4d-831c-87689930a797@amd.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 1/14/26 20:47, Garg, Shivank wrote: > > > On 1/11/2026 4:59 PM, David Hildenbrand (Red Hat) wrote: >> On 1/10/26 19:20, Garg, Shivank wrote: >>> >>> >>> On 1/9/2026 8:16 PM, David Hildenbrand (Red Hat) wrote: >>>> On 12/15/25 09:46, Shivank Garg wrote: > >>>> >>>> This looks a bit complicated. Can't we move that handing up, where we have most of that >>>> information already? Or am I missing something important? >>>> >>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>> index 97d1b2824386f..c7271877c5220 100644 >>>> --- a/mm/khugepaged.c >>>> +++ b/mm/khugepaged.c >>>> @@ -22,6 +22,7 @@ >>>>   #include >>>>   #include >>>>   #include >>>> +#include >>>>     #include >>>>   #include "internal.h" >>>> @@ -2786,7 +2787,9 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, >>>>            for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) { >>>>                  int result = SCAN_FAIL; >>>> +               bool triggered_wb = false; >>>>   +retry: >>>>                  if (!mmap_locked) { >>>>                          cond_resched(); >>>>                          mmap_read_lock(mm); >>>> @@ -2809,6 +2812,16 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start, >>>>                          mmap_locked = false; >>> >>>               *lock_dropped = true; >>>>                          result = hpage_collapse_scan_file(mm, addr, file, pgoff, >>>>                                                            cc); >>>> + >>>> +                       if (result == SCAN_PAGE_DIRTY_OR_WRITEBACK && !triggered_wb && >>>> +                           mapping_can_writeback(file->f_mapping)) { >>>> +                               loff_t lstart = (loff_t)pgoff << PAGE_SHIFT; >>>> +                               loff_t lend = lstart + HPAGE_PMD_SIZE - 1; >>>> + >>>> +                               filemap_write_and_wait_range(file->f_mapping, lstart, lend); >>>> +                               triggered_wb = true; >>> >>>                   fput(file); >>> >>>> +                               goto retry; >>>> +                       } >>>>                          fput(file); >>>>                  } else { >>>>                          result = hpage_collapse_scan_pmd(mm, vma, addr, >>>> >>>> >>> >>> Thank you for the suggestion, this approach looks much simpler. >>> >>> There are two small nits I observed: >> >> Yeah, was a quick untested hack to see if this can be simplified :) >> >>> >>> 1. In the retry loop, it is possible that we reacquire the mmap_lock and set >>>     mmap_locked to true. This can cause issues later when we do: >>> >>>         if (!mmap_locked) >>>                 *lock_dropped = true; >> >> That whole logic of having two variables that express whether locks have been taken/dropped is just absolutely confusing. Any way we can clean that up? >> >>> >>>     because the caller would no longer see that the lock was dropped earlier. >>> >>> 2. We need an fput() to balance the file reference taken at line 2795. >> >> Ah, yes, makes sense. Having a single fput() would be nicer, but that would require yet another temporary variable. >> > > I agree, that this interaction for lock taken/droped is confusing. > However, a proper clean-up would require refactoring the locking logic across multiple functions in the collapse call-flow path. > This seems significantly more invasive and risky. > > I would like to handle this refactoring but in a separate TODO for later. > Could we please proceed with these minimal changes for now? Sure, fine with me. > > Since, V4 has been in the linux-next/mm-unstable for a while, should I send a v5 or an incremental clean-up on top for this? Just send a v4, unless Andrew tells you otherwise :) -- Cheers David