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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 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-Originating-IP: [10.67.120.129] X-ClientProxiedBy: dggems703-chm.china.huawei.com (10.3.19.180) To dggpemf200006.china.huawei.com (7.185.36.61) X-Rspamd-Queue-Id: C7AAF160025 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: s1c3u9um98j13bu7pwc6fcag7wyztwwq X-HE-Tag: 1723691049-388345 X-HE-Meta: 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 EDOp69nD KgHx1kKglGnPvYhKSapUIp1qbtIxVLu5nCj6oegK/e9ESgiY6FIu32A/0XbGuTbFEzy0BAo3VHaELtZvGHOMYIOkM1FZNtlI1/OiR1SOEF2U379aICTxIgyQo1A8Tb37c2tvb7ls0ub8o0HfrXslXnXjA2qHF45B5vmDlKYHRx+iGR9jWwfIfnfrlsEBFEgErzY0z6W4RGIpLoGAIsxITkzWfQtzOi5XGV7rBMdd41v830SSnPbQD81qYZERNzQybju3mFSf21+5EkFWmdi98jzJBT2vFLXV+bw3sx4Yk1iJGqriXgieY3HCD+03vJipANxA3l9uoV5vRnwAUbamKg+lmIg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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.