From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from canpmsgout04.his.huawei.com (canpmsgout04.his.huawei.com [113.46.200.219]) (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 1153F28EA56; Mon, 15 Dec 2025 03:42:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=113.46.200.219 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765770153; cv=none; b=BYXbdt9DDx7Ht4QUWqRyDfv25B0zUAVgnq5Fg7vLtbeROBtNGMAdqy6JBuGh7342wAe2/MgJcn9Vb2hWtw9lazeElHet4L8p4mRlP2Q5B9oDH143F1s3WfyGHfPTER6o716yRzebvXOaEj4jCoe/d3115bbqGR6SfftPRn/leMc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1765770153; c=relaxed/simple; bh=5qgXJr5KAX0ZZRphUl/JuF35O9sLdWYMS5Ca1018yu4=; h=Message-ID:Date:MIME-Version:Subject:From:To:CC:References: In-Reply-To:Content-Type; b=te3yk1BnKVQV51gPhwJSpHR9KI8r+E+SlXZEfo8Ez2vMgL23OsV0MB0vlzPFlVaTUxeMw5FuxGtLehOxPDi45ELvw38jWx+w/JNmVILQXmWNBfyZauR01qwVZTYBMZtMaovCWYc07+VfNL04QqomV4ULjFPQ3cC0hKZa7TcpOE8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b=QrXdkt8w; arc=none smtp.client-ip=113.46.200.219 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b="QrXdkt8w" dkim-signature: v=1; a=rsa-sha256; d=huawei.com; s=dkim; c=relaxed/relaxed; q=dns/txt; h=From; bh=3QQAN60cdddSSldQIQPQz11+trQiMjxdlqc/hEj6Yfg=; b=QrXdkt8w6pPuZ4/WwMhbvSZeCFVKo9LVhCa2tyfo7V8P0FMcddJEzgoZTB9N+1AkB8MNlO7pK X7r80BpTvMe+3Hik4kEoXY8nYlLLICTDVqOHap0jk3zRXS/ReMuZhgU6CLyo0f1r/wcR+3gT1IZ Zmt2E3J7YkwMN+k18SZOi7o= Received: from mail.maildlp.com (unknown [172.19.162.254]) by canpmsgout04.his.huawei.com (SkyGuard) with ESMTPS id 4dV5Rm5hggz1prPH; Mon, 15 Dec 2025 11:40:20 +0800 (CST) Received: from kwepemr500001.china.huawei.com (unknown [7.202.194.229]) by mail.maildlp.com (Postfix) with ESMTPS id 72E58180478; Mon, 15 Dec 2025 11:42:21 +0800 (CST) Received: from [10.174.179.179] (10.174.179.179) by kwepemr500001.china.huawei.com (7.202.194.229) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.11; Mon, 15 Dec 2025 11:42:20 +0800 Message-ID: <164b53af-f4b2-49bf-ac4c-c7b10c080159@huawei.com> Date: Mon, 15 Dec 2025 11:42:19 +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 v4] lib: xarray: free unused spare node in xas_create_range() From: Jinjiang Tu To: Shardul Bankar , , , CC: , , , , , , Kefeng Wang References: <20251204142625.1763372-1-shardul.b@mpiricsoftware.com> <89b96a9f-1d03-440a-93cd-2b9876be3122@huawei.com> In-Reply-To: <89b96a9f-1d03-440a-93cd-2b9876be3122@huawei.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-ClientProxiedBy: kwepems100001.china.huawei.com (7.221.188.238) To kwepemr500001.china.huawei.com (7.202.194.229) 在 2025/12/15 10:19, Jinjiang Tu 写道: > > > 在 2025/12/4 22:26, Shardul Bankar 写道: >> xas_create_range() is typically called in a retry loop that uses >> xas_nomem() to handle -ENOMEM errors. xas_nomem() may allocate a spare >> xa_node and store it in xas->xa_alloc for use in the retry. >> >> If the lock is dropped after xas_nomem(), another thread can expand the >> xarray tree in the meantime. On the next retry, xas_create_range() can >> then succeed without consuming the spare node stored in xas->xa_alloc. >> If the function returns without freeing this spare node, it leaks. >> >> xas_create_range() calls xas_create() multiple times in a loop for >> different index ranges. A spare node that isn't needed for one range >> iteration might be needed for the next, so we cannot free it after each >> xas_create() call. We can only safely free it after xas_create_range() >> completes. >> >> Fix this by calling xas_destroy() at the end of xas_create_range() to >> free any unused spare node. This makes the API safer by default and >> prevents callers from needing to remember cleanup. >> >> This fixes a memory leak in mm/khugepaged.c and potentially other >> callers that use xas_nomem() with xas_create_range(). > I encountered another memory leak issue in xas_create_range(). > > collapse_file() calls xas_create_range() to pre-create all slots needed. > If collapse_file() finally fails, these pre-created slots are empty nodes. > When the file is deleted, shmem_evict_inode()->shmem_truncate_range()->shmem_undo_range() > calls xas_store(&xas, NULL) for each entries to delete nodes, but leaving those pre-created > empty nodes leaked. > > I can reproduce it with following steps. > 1) create file /tmp/test_madvise_collapse and ftruncate to 4MB size, and then mmap the file > 2) memset for the first 2MB > 3) madvise(MADV_COLLAPSE) for the second 2MB. > 4) unlink the file > > in 3), collapse_file() calls xas_create_range() to expand xarray depth, and fails to collapse > due to the whole 2M region is empty, leading to the new created empty nodes leaked. > > To fix it, maybe we should add a new function xas_delete_range() to revert what xas_create_range() > does when xas_create_range() runs into rollback path? How about the following diff? I tried it, and the memory leak disappears. I'm new in xarray, so I don't if this fix works properly. diff --git a/include/linux/xarray.h b/include/linux/xarray.h index be850174e802..972df5ceeb84 100644 --- a/include/linux/xarray.h +++ b/include/linux/xarray.h @@ -1555,6 +1555,7 @@ void xas_destroy(struct xa_state *);  void xas_pause(struct xa_state *);  void xas_create_range(struct xa_state *); +void xas_destroy_range(struct xa_state *xas, unsigned long start, unsigned long end);  #ifdef CONFIG_XARRAY_MULTI  int xa_get_order(struct xarray *, unsigned long index); diff --git a/lib/xarray.c b/lib/xarray.c index 9a8b4916540c..ab15dc939962 100644 --- a/lib/xarray.c +++ b/lib/xarray.c @@ -752,6 +752,21 @@ void xas_create_range(struct xa_state *xas)  }  EXPORT_SYMBOL_GPL(xas_create_range); +void xas_destroy_range(struct xa_state *xas, unsigned long start, unsigned long end) +{ +    unsigned long index; +    void *entry; + +    for (index = start; index < end; ++index) { +        xas_set(xas, index); +        entry = xas_load(xas); +        if (entry) +            continue; +        else if (xas->xa_node && !xas->xa_node->count) +            xas_delete_node(xas); +    } +} +  static void update_node(struct xa_state *xas, struct xa_node *node,          int count, int values)  { diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 97d1b2824386..dd9d3f202c4b 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -2247,7 +2247,10 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,      goto out;  rollback: -    /* Something went wrong: roll back page cache changes */ +    /* Something went wrong: roll back empty xa_node created by +    * xas_create_range() and page cache changes +    */ +    xas_destroy_range(&xas, start, end); +      if (nr_none) {          xas_lock_irq(&xas);          mapping->nrpages -= nr_none; >> Link:https://syzkaller.appspot.com/bug?id=a274d65fc733448ed518ad15481ed575669dd98c >> Link:https://lore.kernel.org/all/20251201074540.3576327-1-shardul.b@mpiricsoftware.com/ ("v3") >> Fixes: cae106dd67b9 ("mm/khugepaged: refactor collapse_file control flow") >> Signed-off-by: Shardul Bankar >> --- >> v4: >> - Drop redundant `if (xa_alloc)` around xas_destroy(), as xas_destroy() >> already checks xa_alloc internally. >> v3: >> - Move fix from collapse_file() to xas_create_range() as suggested by Matthew Wilcox >> - Fix in library function makes API safer by default, preventing callers from needing >> to remember cleanup >> - Use shared cleanup label that both restore: and success: paths jump to >> - Clean up unused spare node on both success and error exit paths >> v2: >> - Call xas_destroy() on both success and failure >> - Explained retry semantics and xa_alloc / concurrency risk >> - Dropped cleanup_empty_nodes from previous proposal >> >> lib/xarray.c | 7 ++++++- >> 1 file changed, 6 insertions(+), 1 deletion(-) >> >> diff --git a/lib/xarray.c b/lib/xarray.c >> index 9a8b4916540c..f49ccfa5f57d 100644 >> --- a/lib/xarray.c >> +++ b/lib/xarray.c >> @@ -744,11 +744,16 @@ void xas_create_range(struct xa_state *xas) >> xas->xa_shift = shift; >> xas->xa_sibs = sibs; >> xas->xa_index = index; >> - return; >> + goto cleanup; >> + >> success: >> xas->xa_index = index; >> if (xas->xa_node) >> xas_set_offset(xas); >> + >> +cleanup: >> + /* Free any unused spare node from xas_nomem() */ >> + xas_destroy(xas); >> } >> EXPORT_SYMBOL_GPL(xas_create_range); >>