From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-113.freemail.mail.aliyun.com (out30-113.freemail.mail.aliyun.com [115.124.30.113]) (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 C89AE1D7E42 for ; Thu, 17 Jul 2025 08:46:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.113 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752742015; cv=none; b=bFJysEjSrVYII1Tpg4oAiWz09wIII0QuYEm7wJxFt/PC5OSfQN0M5J/PPcdhsZPejJXfOQIFJ4bt0v5VTPVCVG2Z0DDCi1F6zhjFa20greBYGLsIM8JREcNzBuMfZaF1djnhEPoosd+88GowkD+Js1scCFya9fRBxKUc/7TRHaQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752742015; c=relaxed/simple; bh=FXLJcFsoj/SCw8PZIalLLWNq1LUPV8jdyQ175kpC898=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=OfV+MvdDVatTP+LiAxUTH2X7IjptuVErpZdhWIbWw6h44lzcnfnMKJagsHhxgfsUJqAlImwrUEfwRzBYbPd6XYo5s9nmDhheccV8YZgCyD8+niOcITjMwU8xAGkgbxGdLDY7t0c/2zMlhewddJEQ9a+dogOBY8DsmB7DyLpaVJk= 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=bb/wgZXB; arc=none smtp.client-ip=115.124.30.113 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="bb/wgZXB" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1752742008; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=WI9t0vYUiM6EPcwXB+Epv9DnfmRhxSqBDsbok8GgZlc=; b=bb/wgZXBOOKEwCePdVKC8erk5R4LYFbktny4A58m8I6lRoymh+aCESdejS/YUCijQI1xN7Q2wrOJfhZWENAcm+oXVWyX06OVlSiVXYLo8xUO6MFQgMx4T4/DyfGzmvKWr+asLLVAhyCQsn0GKZ/0emOmHJ3UNefLNWoLSTqCRxQ= Received: from 30.74.144.120(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0Wj7R2iT_1752742007 cluster:ay36) by smtp.aliyun-inc.com; Thu, 17 Jul 2025 16:46:48 +0800 Message-ID: Date: Thu, 17 Jul 2025 16:46:47 +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 mm-new 1/2] mm/shmem: hold shmem_swaplist spinlock (not mutex) much less To: Hugh Dickins , Andrew Morton Cc: Baoquan He , Barry Song <21cnbao@gmail.com>, Chris Li , David Rientjes , Kairui Song , Kemeng Shi , Shakeel Butt , linux-kernel@vger.kernel.org, linux-mm@kvack.org References: <87beaec6-a3b0-ce7a-c892-1e1e5bd57aa3@google.com> From: Baolin Wang In-Reply-To: <87beaec6-a3b0-ce7a-c892-1e1e5bd57aa3@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2025/7/16 16:05, Hugh Dickins wrote: > A flamegraph (from an MGLRU load) showed shmem_writeout()'s use of the > global shmem_swaplist_mutex worryingly hot: improvement is long overdue. > > 3.1 commit 6922c0c7abd3 ("tmpfs: convert shmem_writepage and enable swap") > apologized for extending shmem_swaplist_mutex across add_to_swap_cache(), > and hoped to find another way: yes, there may be lots of work to allocate > radix tree nodes in there. Then 6.15 commit b487a2da3575 ("mm, swap: > simplify folio swap allocation") will have made it worse, by moving > shmem_writeout()'s swap allocation under that mutex too (but the worrying > flamegraph was observed even before that change). > > There's a useful comment about pagelock no longer protecting from eviction > once moved to swap cache: but it's good till shmem_delete_from_page_cache() > replaces page pointer by swap entry, so move the swaplist add between them. > > We would much prefer to take the global lock once per inode than once per > page: given the possible races with shmem_unuse() pruning when !swapped > (and other tasks racing to swap other pages out or in), try the swaplist > add whenever swapped was incremented from 0 (but inode may already be on > the list - only unuse and evict bother to remove it). > > This technique is more subtle than it looks (we're avoiding the very lock > which would make it easy), but works: whereas an unlocked list_empty() > check runs a risk of the inode being unqueued and left off the swaplist > forever, swapoff only completing when the page is faulted in or removed. > > The need for a sleepable mutex went away in 5.1 commit b56a2d8af914 > ("mm: rid swapoff of quadratic complexity"): a spinlock works better now. > > This commit is certain to take shmem_swaplist_mutex out of contention, > and has been seen to make a practical improvement (but there is likely > to have been an underlying issue which made its contention so visible). > > Signed-off-by: Hugh Dickins Makes sense to me. Thanks. Reviewed-by: Baolin Wang Tested-by: Baolin Wang > --- > mm/shmem.c | 59 ++++++++++++++++++++++++++++++------------------------ > 1 file changed, 33 insertions(+), 26 deletions(-) > > diff --git a/mm/shmem.c b/mm/shmem.c > index 60247dc48505..33675361031b 100644 > --- a/mm/shmem.c > +++ b/mm/shmem.c > @@ -292,7 +292,7 @@ bool vma_is_shmem(struct vm_area_struct *vma) > } > > static LIST_HEAD(shmem_swaplist); > -static DEFINE_MUTEX(shmem_swaplist_mutex); > +static DEFINE_SPINLOCK(shmem_swaplist_lock); > > #ifdef CONFIG_TMPFS_QUOTA > > @@ -432,10 +432,13 @@ static void shmem_free_inode(struct super_block *sb, size_t freed_ispace) > * > * But normally info->alloced == inode->i_mapping->nrpages + info->swapped > * So mm freed is info->alloced - (inode->i_mapping->nrpages + info->swapped) > + * > + * Return: true if swapped was incremented from 0, for shmem_writeout(). > */ > -static void shmem_recalc_inode(struct inode *inode, long alloced, long swapped) > +static bool shmem_recalc_inode(struct inode *inode, long alloced, long swapped) > { > struct shmem_inode_info *info = SHMEM_I(inode); > + bool first_swapped = false; > long freed; > > spin_lock(&info->lock); > @@ -450,8 +453,11 @@ static void shmem_recalc_inode(struct inode *inode, long alloced, long swapped) > * to stop a racing shmem_recalc_inode() from thinking that a page has > * been freed. Compensate here, to avoid the need for a followup call. > */ > - if (swapped > 0) > + if (swapped > 0) { > + if (info->swapped == swapped) > + first_swapped = true; > freed += swapped; > + } > if (freed > 0) > info->alloced -= freed; > spin_unlock(&info->lock); > @@ -459,6 +465,7 @@ static void shmem_recalc_inode(struct inode *inode, long alloced, long swapped) > /* The quota case may block */ > if (freed > 0) > shmem_inode_unacct_blocks(inode, freed); > + return first_swapped; > } > > bool shmem_charge(struct inode *inode, long pages) > @@ -1399,11 +1406,11 @@ static void shmem_evict_inode(struct inode *inode) > /* Wait while shmem_unuse() is scanning this inode... */ > wait_var_event(&info->stop_eviction, > !atomic_read(&info->stop_eviction)); > - mutex_lock(&shmem_swaplist_mutex); > + spin_lock(&shmem_swaplist_lock); > /* ...but beware of the race if we peeked too early */ > if (!atomic_read(&info->stop_eviction)) > list_del_init(&info->swaplist); > - mutex_unlock(&shmem_swaplist_mutex); > + spin_unlock(&shmem_swaplist_lock); > } > } > > @@ -1526,7 +1533,7 @@ int shmem_unuse(unsigned int type) > if (list_empty(&shmem_swaplist)) > return 0; > > - mutex_lock(&shmem_swaplist_mutex); > + spin_lock(&shmem_swaplist_lock); > start_over: > list_for_each_entry_safe(info, next, &shmem_swaplist, swaplist) { > if (!info->swapped) { > @@ -1540,12 +1547,12 @@ int shmem_unuse(unsigned int type) > * (igrab() would protect from unlink, but not from unmount). > */ > atomic_inc(&info->stop_eviction); > - mutex_unlock(&shmem_swaplist_mutex); > + spin_unlock(&shmem_swaplist_lock); > > error = shmem_unuse_inode(&info->vfs_inode, type); > cond_resched(); > > - mutex_lock(&shmem_swaplist_mutex); > + spin_lock(&shmem_swaplist_lock); > if (atomic_dec_and_test(&info->stop_eviction)) > wake_up_var(&info->stop_eviction); > if (error) > @@ -1556,7 +1563,7 @@ int shmem_unuse(unsigned int type) > if (!info->swapped) > list_del_init(&info->swaplist); > } > - mutex_unlock(&shmem_swaplist_mutex); > + spin_unlock(&shmem_swaplist_lock); > > return error; > } > @@ -1646,30 +1653,30 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug, > folio_mark_uptodate(folio); > } > > - /* > - * Add inode to shmem_unuse()'s list of swapped-out inodes, > - * if it's not already there. Do it now before the folio is > - * moved to swap cache, when its pagelock no longer protects > - * the inode from eviction. But don't unlock the mutex until > - * we've incremented swapped, because shmem_unuse_inode() will > - * prune a !swapped inode from the swaplist under this mutex. > - */ > - mutex_lock(&shmem_swaplist_mutex); > - if (list_empty(&info->swaplist)) > - list_add(&info->swaplist, &shmem_swaplist); > - > if (!folio_alloc_swap(folio, __GFP_HIGH | __GFP_NOMEMALLOC | __GFP_NOWARN)) { > - shmem_recalc_inode(inode, 0, nr_pages); > + bool first_swapped = shmem_recalc_inode(inode, 0, nr_pages); > + > + /* > + * Add inode to shmem_unuse()'s list of swapped-out inodes, > + * if it's not already there. Do it now before the folio is > + * removed from page cache, when its pagelock no longer > + * protects the inode from eviction. And do it now, after > + * we've incremented swapped, because shmem_unuse() will > + * prune a !swapped inode from the swaplist. > + */ > + if (first_swapped) { > + spin_lock(&shmem_swaplist_lock); > + if (list_empty(&info->swaplist)) > + list_add(&info->swaplist, &shmem_swaplist); > + spin_unlock(&shmem_swaplist_lock); > + } > + > swap_shmem_alloc(folio->swap, nr_pages); > shmem_delete_from_page_cache(folio, swp_to_radix_entry(folio->swap)); > > - mutex_unlock(&shmem_swaplist_mutex); > BUG_ON(folio_mapped(folio)); > return swap_writeout(folio, plug); > } > - if (!info->swapped) > - list_del_init(&info->swaplist); > - mutex_unlock(&shmem_swaplist_mutex); > if (nr_pages > 1) > goto try_split; > redirty: