From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from szxga02-in.huawei.com (szxga02-in.huawei.com [45.249.212.188]) (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 5E23B18641; Thu, 15 Aug 2024 03:04:08 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=45.249.212.188 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723691051; cv=none; b=e25OKyjmZUJp29SFgX3fefMG/sdWCsVA9okenViZrFQZ29607Tmxcz2ol3HKSuxb8HdLX+z3rJG8x5aOVqhYdejY9lSE6iYBWKMxIqc6mS0WBHpyOra+JraVj63LaOHkEz8dh/Y60ksXwwepKpO++2xXHzcH8n4cgo9kkbFjXyM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1723691051; c=relaxed/simple; bh=gcpoaIIxeNLcQfhBxNRy+vr76gHVNnEZRZkhQvVZjtg=; h=Message-ID:Date:MIME-Version:Subject:To:CC:References:From: In-Reply-To:Content-Type; b=vCimSzGkUm5H5Ik3jD0i71o9ji9P4AMvsRl9lnzC/2NSkZSyBD019KmAAQUp6BQHg8ympLi4HQQsr4fqE0b7IciwodXdlHo2pwY9cB8o9/odMuMyiYnlyjOjvuwdCMkTSdT+o+XLPDhqy0qDE9nNtDFvwEoRxhB+NWg9ttgLu0U= 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; arc=none smtp.client-ip=45.249.212.188 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 Received: from mail.maildlp.com (unknown [172.19.163.252]) by szxga02-in.huawei.com (SkyGuard) with ESMTP id 4Wkqg85bLQzpSwK; Thu, 15 Aug 2024 11:02:44 +0800 (CST) Received: from dggpemf200006.china.huawei.com (unknown [7.185.36.61]) by mail.maildlp.com (Postfix) with ESMTPS id 8DF5A1800A5; Thu, 15 Aug 2024 11:04:06 +0800 (CST) Received: from [10.67.120.129] (10.67.120.129) by dggpemf200006.china.huawei.com (7.185.36.61) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.11; Thu, 15 Aug 2024 11:04:06 +0800 Message-ID: <82cc55f0-35e9-4e54-8316-00312389de3f@huawei.com> Date: Thu, 15 Aug 2024 11:04:06 +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 net-next v13 08/14] mm: page_frag: some minor refactoring before adding new API To: Alexander H Duyck , , , CC: , , Andrew Morton , References: <20240808123714.462740-1-linyunsheng@huawei.com> <20240808123714.462740-9-linyunsheng@huawei.com> <7d16ba784eb564f9d556f532d670b9bc4698d913.camel@gmail.com> Content-Language: en-US From: Yunsheng Lin In-Reply-To: <7d16ba784eb564f9d556f532d670b9bc4698d913.camel@gmail.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To dggpemf200006.china.huawei.com (7.185.36.61) On 2024/8/15 1:54, Alexander H Duyck wrote: > On Thu, 2024-08-08 at 20:37 +0800, Yunsheng Lin wrote: >> Refactor common codes from __page_frag_alloc_va_align() >> to __page_frag_cache_reload(), so that the new API can >> make use of them. >> >> CC: Alexander Duyck >> Signed-off-by: Yunsheng Lin >> --- >> include/linux/page_frag_cache.h | 2 +- >> mm/page_frag_cache.c | 138 ++++++++++++++++++-------------- >> 2 files changed, 81 insertions(+), 59 deletions(-) >> >> diff --git a/include/linux/page_frag_cache.h b/include/linux/page_frag_cache.h >> index 4ce924eaf1b1..0abffdd10a1c 100644 >> --- a/include/linux/page_frag_cache.h >> +++ b/include/linux/page_frag_cache.h >> @@ -52,7 +52,7 @@ static inline void *encoded_page_address(unsigned long encoded_va) >> >> static inline void page_frag_cache_init(struct page_frag_cache *nc) >> { >> - nc->encoded_va = 0; >> + memset(nc, 0, sizeof(*nc)); >> } >> > > Still not a fan of this. Just setting encoded_va to 0 should be enough > as the other fields will automatically be overwritten when the new page > is allocated. > > Relying on memset is problematic at best since you then introduce the > potential for issues where remaining somehow gets corrupted but > encoded_va/page is 0. I would rather have both of these being checked > as a part of allocation than just just assuming it is valid if > remaining is set. Does adding something like VM_BUG_ON(!nc->encoded_va && nc->remaining) to catch the above problem address your above concern? > > I would prefer to keep the check for a non-0 encoded_page value and > then check remaining rather than just rely on remaining as it creates a > single point of failure. With that we can safely tear away a page and > the next caller to try to allocate will populated a new page and the > associated fields. As mentioned before, the memset() is used mainly because of: 1. avoid a checking in the fast path. 2. avoid duplicating the checking pattern you mentioned above for the new API. > >> static inline bool page_frag_cache_is_pfmemalloc(struct page_frag_cache *nc) >> diff --git a/mm/page_frag_cache.c b/mm/page_frag_cache.c >> index 2544b292375a..4e6b1c4684f0 100644 >> --- a/mm/page_frag_cache.c >> +++ b/mm/page_frag_cache.c >> @@ -19,8 +19,27 @@ >> #include >> #include "internal.h" >> ... >> + >> +/* Reload cache by reusing the old cache if it is possible, or >> + * refilling from the page allocator. >> + */ >> +static bool __page_frag_cache_reload(struct page_frag_cache *nc, >> + gfp_t gfp_mask) >> +{ >> + if (likely(nc->encoded_va)) { >> + if (__page_frag_cache_reuse(nc->encoded_va, nc->pagecnt_bias)) >> + goto out; >> + } >> + >> + if (unlikely(!__page_frag_cache_refill(nc, gfp_mask))) >> + return false; >> + >> +out: >> + /* reset page count bias and remaining to start of new frag */ >> + nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1; >> + nc->remaining = page_frag_cache_page_size(nc->encoded_va); > > One thought I am having is that it might be better to have the > pagecnt_bias get set at the same time as the page_ref_add or the > set_page_count call. In addition setting the remaining value at the > same time probably would make sense as in the refill case you can make > use of the "order" value directly instead of having to write/read it > out of the encoded va/page. Probably, there is always tradeoff to make regarding avoid code duplication and avoid reading the order, I am not sure it matters for both for case, I would rather keep the above pattern if there is not obvious benefit for the other pattern. > > With that we could simplify this function and get something closer to > what we had for the original alloc_va_align code. > >> + return true; >> } >> >> void page_frag_cache_drain(struct page_frag_cache *nc) >> @@ -55,7 +100,7 @@ void page_frag_cache_drain(struct page_frag_cache *nc) >> >> __page_frag_cache_drain(virt_to_head_page((void *)nc->encoded_va), >> nc->pagecnt_bias); >> - nc->encoded_va = 0; >> + memset(nc, 0, sizeof(*nc)); >> } >> EXPORT_SYMBOL(page_frag_cache_drain); >> >> @@ -73,67 +118,44 @@ void *__page_frag_alloc_va_align(struct page_frag_cache *nc, >> unsigned int align_mask) >> { >> unsigned long encoded_va = nc->encoded_va; >> - unsigned int size, remaining; >> - struct page *page; >> - >> - if (unlikely(!encoded_va)) { > > We should still be checking this before we even touch remaining. > Otherwise we greatly increase the risk of providing a bad virtual > address and have greatly decreased the likelihood of us catching > potential errors gracefully. > >> -refill: >> - page = __page_frag_cache_refill(nc, gfp_mask); >> - if (!page) >> - return NULL; >> - >> - encoded_va = nc->encoded_va; >> - size = page_frag_cache_page_size(encoded_va); >> - >> - /* Even if we own the page, we do not use atomic_set(). >> - * This would break get_page_unless_zero() users. >> - */ >> - page_ref_add(page, PAGE_FRAG_CACHE_MAX_SIZE); >> - >> - /* reset page count bias and remaining to start of new frag */ >> - nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1; >> - nc->remaining = size; > > With my suggested change above you could essentially just drop the > block starting from the comment and this function wouldn't need to > change as much as it is. It seems you are still suggesting that new API also duplicates the old checking pattern in __page_frag_alloc_va_align()? I would rather avoid the above if something like VM_BUG_ON() can address your above concern.