From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-vs1-f51.google.com (mail-vs1-f51.google.com [209.85.217.51]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8E05F13F420 for ; Fri, 26 Apr 2024 09:46:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.217.51 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714124777; cv=none; b=rgUlunrUHpxnV2gzBT+qhYWKaA8xXQZaEB0wRrxOUt19CwSGbUugNadWqxUz5nPVhmj44Elwcf2IIE0ZFxD6IETqaraUjak0Gt5D5U1pZCZ1v73N4VXcGeVkgDAZq4QO6an7p4nsG0N+dQbkotWGJLuwT2eilx/TjLMISgqBJZ0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714124777; c=relaxed/simple; bh=jXM7Ok6jB6oCe4y9joHPmVL04bb4gFV5x/qKAN2nhEw=; h=MIME-Version:References:In-Reply-To:From:Date:Message-ID:Subject: To:Cc:Content-Type; b=OsfhQDDTWLQgGxBp8qUxJjYMrGiPhbPgr0bgWC4BdZDDHKUpHXt15Ryg9+HCLbwsHh/wv7bXFDRXAPUTuxpyHiTnOjvU1uwh2uiQyA7WPNLPhe9sV51bde3RKIp/OvHcXHuZ1cfEATzZoi5RsHsflZXfXnLsH28wHWuQ9GM4OkY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=irggxz+O; arc=none smtp.client-ip=209.85.217.51 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="irggxz+O" Received: by mail-vs1-f51.google.com with SMTP id ada2fe7eead31-479e12bde9fso823764137.2 for ; Fri, 26 Apr 2024 02:46:15 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1714124774; x=1714729574; darn=vger.kernel.org; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:from:to:cc:subject:date :message-id:reply-to; bh=KVOougTKR2E63G5I1kT79Wg3Gm+lmkU4LrYVubtdS5o=; b=irggxz+OzlsZ+ozLr+2hJwV16ByA0Zjt9n9VDy0z9b6HXYZHR8CFbiBrlyRY7JUloJ j3jkUggj7cnOWfavyYroJ76jvaBOlpeZnRBQFKcyYleymhRvXcmXHSKDoWGSgtCr3VLx HoAtbfGt/F+j/4NWePxWK4QTfpIzq1nHVqR3GDcwtP/3Of9/9YjL1JqaLMdO3gfXWgGo aBaI0dPL1e3AOQc/Vv/Ftbx4542/tyDPeSyjosOMY3/PBjlV+VGGeB4iGkqvcIlAdyIe 5ZjksKwmJf6VrVig4Qsc+CYi8KKxuod7/32iK9EMi/R4sbUSwaWTi4MVNUmLbLU19q32 bI5Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1714124774; x=1714729574; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=KVOougTKR2E63G5I1kT79Wg3Gm+lmkU4LrYVubtdS5o=; b=YF4qC0h6lwrmRVwJUQyG6Jy1cjcLPd1eCu+k03+NFrPqZiYU/ZSaNl0yw7FzcfXg8t TyPfOl0MrYRZDcUlhnBtXloKxrwFrtd8vX62oSB5Gc2/zqu1pn8NkpLfzFQ4j19/0tOz Mx901eIBLMVQz8gdN0CQf1Dij+U8XCyuXV4Lmw/UaLTmu8xW0rotV6nQnxjQM9L8sBtp AYkEmf0JKuOiZOUDDAahkICZTSspP/Seek/byvK53njFbc6BQRUSX+1CBS/P9C5ROu/6 7dsdD2p/iP0r1/h/5PVCZnTiCINhlosCQBDTmViw4y9mAUfcODTPOk++qOVhcFQfffXi +DlA== X-Forwarded-Encrypted: i=1; AJvYcCXa9EdlnMKIh2LN651DaTtbBpG/fC4lQ0eRGj+LpX9GGQoVsNud5mXXGqHW+kVW837aia4qPcY4nJvpNuWeviUYWe/hyPPTpk2NuS1c X-Gm-Message-State: AOJu0YzNrWIjGK4TvY4eTGk9432TPJc+cKhZt0hm21rNjRtgSpOamZSU HO6YzSTito70YgcLDFeNra8L9OnBBVaXo2IjUME33DPFdH+aqcJEJeoedN/nhjgTI31NblZ04V+ k+CddMGoK23LwtCuIPXjO0ChzUKo= X-Google-Smtp-Source: AGHT+IH58NJkrUWm0u8dX1JxeSV3i50SJdOXfy/TMRuL7cqe2OlgLT61w+ULPg2fuH1Bw+QOfGAUGozg/klLtLMqFyw= X-Received: by 2002:a05:6102:6d1:b0:47b:976d:8374 with SMTP id m17-20020a05610206d100b0047b976d8374mr2487387vsg.29.1714124774146; Fri, 26 Apr 2024 02:46:14 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20240425211136.486184-1-zi.yan@sent.com> In-Reply-To: From: Barry Song <21cnbao@gmail.com> Date: Fri, 26 Apr 2024 17:46:02 +0800 Message-ID: Subject: Re: [PATCH v4] mm/rmap: do not add fully unmapped large folio to deferred split list To: David Hildenbrand Cc: Zi Yan , Andrew Morton , linux-mm@kvack.org, "Matthew Wilcox (Oracle)" , Yang Shi , Ryan Roberts , Lance Yang , linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Fri, Apr 26, 2024 at 4:19=E2=80=AFPM David Hildenbrand wrote: > > On 25.04.24 23:11, Zi Yan wrote: > > From: Zi Yan > > > > In __folio_remove_rmap(), a large folio is added to deferred split list > > if any page in a folio loses its final mapping. But it is possible that > > the folio is fully unmapped and adding it to deferred split list is > > unnecessary. > > > > For PMD-mapped THPs, that was not really an issue, because removing the > > last PMD mapping in the absence of PTE mappings would not have added th= e > > folio to the deferred split queue. > > > > However, for PTE-mapped THPs, which are now more prominent due to mTHP, > > they are always added to the deferred split queue. One side effect > > is that the THP_DEFERRED_SPLIT_PAGE stat for a PTE-mapped folio can be > > unintentionally increased, making it look like there are many partially > > mapped folios -- although the whole folio is fully unmapped stepwise. > > > > Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped THPs > > where possible starting from commit b06dc281aa99 ("mm/rmap: introduce > > folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE-mapp= ed > > folio is unmapped in one go and can avoid being added to deferred split > > list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will still = be > > noise when we cannot batch-unmap a complete PTE-mapped folio in one go > > -- or where this type of batching is not implemented yet, e.g., migrati= on. > > > > To avoid the unnecessary addition, folio->_nr_pages_mapped is checked > > to tell if the whole folio is unmapped. If the folio is already on > > deferred split list, it will be skipped, too. > > > > Note: commit 98046944a159 ("mm: huge_memory: add the missing > > folio_test_pmd_mappable() for THP split statistics") tried to exclude > > mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not > > fix the above issue. A fully unmapped PTE-mapped order-9 THP was still > > added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE, > > since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside > > deferred_split_folio() the order-9 folio is folio_test_pmd_mappable(). > > > > Signed-off-by: Zi Yan > > Reviewed-by: Yang Shi > > --- > > mm/rmap.c | 8 +++++--- > > 1 file changed, 5 insertions(+), 3 deletions(-) > > > > diff --git a/mm/rmap.c b/mm/rmap.c > > index a7913a454028..220ad8a83589 100644 > > --- a/mm/rmap.c > > +++ b/mm/rmap.c > > @@ -1553,9 +1553,11 @@ static __always_inline void __folio_remove_rmap(= struct folio *folio, > > * page of the folio is unmapped and at least one page > > * is still mapped. > > */ > > - if (folio_test_large(folio) && folio_test_anon(folio)) > > - if (level =3D=3D RMAP_LEVEL_PTE || nr < nr_pmdmap= ped) > > - deferred_split_folio(folio); > > + if (folio_test_large(folio) && folio_test_anon(folio) && > > + list_empty(&folio->_deferred_list) && > > + ((level =3D=3D RMAP_LEVEL_PTE && atomic_read(mapped))= || > > + (level =3D=3D RMAP_LEVEL_PMD && nr < nr_pmdmapped))) > > + deferred_split_folio(folio); > > } > > > > /* > > > > base-commit: 66313c66dd90e8711a8b63fc047ddfc69c53636a > > Reviewed-by: David Hildenbrand > > But maybe we can really improve the code: > > > diff --git a/mm/rmap.c b/mm/rmap.c > index 2608c40dffade..e310b6c4221d7 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1495,6 +1495,7 @@ static __always_inline void __folio_remove_rmap(str= uct folio *folio, > { > atomic_t *mapped =3D &folio->_nr_pages_mapped; > int last, nr =3D 0, nr_pmdmapped =3D 0; > + bool partially_mapped =3D false; > enum node_stat_item idx; > > __folio_rmap_sanity_checks(folio, page, nr_pages, level); > @@ -1515,6 +1516,8 @@ static __always_inline void __folio_remove_rmap(str= uct folio *folio, > nr++; > } > } while (page++, --nr_pages > 0); > + > + partially_mapped =3D nr && atomic_read(mapped); nice! > break; > case RMAP_LEVEL_PMD: > atomic_dec(&folio->_large_mapcount); > @@ -1532,6 +1535,7 @@ static __always_inline void __folio_remove_rmap(str= uct folio *folio, > nr =3D 0; > } > } > + partially_mapped =3D nr < nr_pmdmapped; > break; > } > > @@ -1553,9 +1557,9 @@ static __always_inline void __folio_remove_rmap(str= uct folio *folio, > * page of the folio is unmapped and at least one page > * is still mapped. > */ > - if (folio_test_large(folio) && folio_test_anon(folio)) > - if (level =3D=3D RMAP_LEVEL_PTE || nr < nr_pmdmap= ped) > - deferred_split_folio(folio); > + if (folio_test_large(folio) && folio_test_anon(folio) && > + list_empty(&folio->_deferred_list) && partially_mappe= d) > + deferred_split_folio(folio); > } > > /* > > The compiler should be smart enough to optimize it all -- most likely ;) > > -- > Cheers, > > David / dhildenb >