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linux- > hyperv@vger.kernel.org; akpm@linux-foundation.org; linux-mm@kvack.org > Cc: Dexuan Cui ; KY Srinivasan ; > Paul Rosswurm ; olaf@aepfle.de; vkuznets > ; davem@davemloft.net; wei.liu@kernel.org; Long Li > ; linux-kernel@vger.kernel.org; > linyunsheng@huawei.com; stable@vger.kernel.org; netdev@vger.kernel.org; > Alexander Duyck > Subject: RE: [EXTERNAL] Re: [PATCH] mm: page_frag: Fix refill handling in > __page_frag_alloc_align() > > > > > -----Original Message----- > > From: Yunsheng Lin > > Sent: Saturday, March 1, 2025 8:50 AM > > To: Haiyang Zhang ; linux- > hyperv@vger.kernel.org; > > akpm@linux-foundation.org; linux-mm@kvack.org > > Cc: Dexuan Cui ; KY Srinivasan = ; > > Paul Rosswurm ; olaf@aepfle.de; vkuznets > > ; davem@davemloft.net; wei.liu@kernel.org; Long Li > > ; linux-kernel@vger.kernel.org; > > linyunsheng@huawei.com; stable@vger.kernel.org; netdev@vger.kernel.org; > > Alexander Duyck > > Subject: [EXTERNAL] Re: [PATCH] mm: page_frag: Fix refill handling in > > __page_frag_alloc_align() > > > > +cc netdev ML & Alexander > > > > On 3/1/2025 10:03 AM, Haiyang Zhang wrote: > > > In commit 8218f62c9c9b ("mm: page_frag: use initial zero offset for > > > page_frag_alloc_align()"), the check for fragsz is moved earlier. > > > So when the cache is used up, and if the fragsz > PAGE_SIZE, it won't > > > try to refill, and just return NULL. > > > I tested it with fragsz:8192, cache-size:32768. After the initial fou= r > > > successful allocations, it failed, even there is plenty of free memor= y > > > in the system. > > > > Hi, Haiyang > > It seems the PAGE_SIZE is 4K for the tested system? > Yes. > > > Which drivers or subsystems are passing the fragsz being bigger than > > PAGE_SIZE to page_frag_alloc_align() related API? > For example, our MANA driver when using jumbo frame. > https://web.git/. > kernel.org%2Fpub%2Fscm%2Flinux%2Fkernel%2Fgit%2Fnetdev%2Fnet- > next.git%2Ftree%2Fdrivers%2Fnet%2Fethernet%2Fmicrosoft%2Fmana&data=3D05%7= C02 > %7Chaiyangz%40microsoft.com%7Cea9cc3de8c904a5c720408dd58e2913a%7C72f988bf= 8 > 6f141af91ab2d7cd011db47%7C1%7C0%7C638764452327076527%7CUnknown%7CTWFpbGZs= b > 3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTW= F > pbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=3D7%2BZ9hBYGudGZUeeF0i4UEa3zjx4Z= LFd > q5E3qcZxnIWE%3D&reserved=3D0 > > > > To fix, revert the refill logic like before: the refill is attempted > > > before the check & return NULL. > > > > page_frag API is not really for allocating memory being bigger than > > PAGE_SIZE as __page_frag_cache_refill() will not try hard enough to > > allocate order 3 compound page when calling __alloc_pages() and will > > fail back to allocate base page as the discussed in below: > > > https://lore.ker/ > %2F&data=3D05%7C02%7Chaiyangz%40microsoft.com%7Cea9cc3de8c904a5c720408dd5= 8e2 > 913a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C638764452327105287%7CUn= k > nown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW= 4 > zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=3Dg4VAI8DbzUD95q= gth > vzFV0PYgOIA3%2F%2FI3gmQHzuLwbo%3D&reserved=3D0 > > nel.org%2Fall%2Fead00fb7-8538-45b3-8322- > > > 8a41386e7381%40huawei.com%2F&data=3D05%7C02%7Chaiyangz%40microsoft.com%7C= d73 > > > d6a0ae65b4a42681c08dd58c8087b%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%= 7 > > > C638764338396356411%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiO= i > > > IwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%= 7 > > C&sdata=3DFJ7Ggrxxxv6QzKepUiHmtns1GZC2G2oJMcWSzOuFbsE%3D&reserved=3D0 > We are already aware of this, and have error checking in place for the > failover > case to "base page". > > From the discussion thread above, there are other drivers using > page_frag_alloc_align() for over PAGE_SIZE too. If making the page_frag > API > support only fragsz <=3D PAGE_SIZE is desired, can we create another API? > One > keeps the existing API semantics (allowing > PAGE_SIZE), the other uses > your new code. By the way, it should add an explicit check and fail ALL > requests > for fragsz > PAGE_SIZE. Currently your code successfully allocates big > frags > for a few times, then fail. This is not a desired behavior. It's also a > breaking change for our MANA driver, which can no longer run Jumbo frames= . > > @Andrew Morton > And other maintainers, could you please also evaluate the idea above? > And, quote from current doc 6.14.0-rc4: "A page fragment is an arbitrary-length arbitrary-offset area of memory which resides within a 0 or higher order compound page." https://web.git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/tre= e/Documentation/mm/page_frags.rst So, it is designed to be *arbitrary-length* within a 0 or higher order compound page. If the commit 8218f62c9c9b ("mm: page_frag: use initial zero offset for page_frag_alloc_align()") intended to change the existing API semantics to be Page Frag Length <=3D PAGE_SIZE, the document and all breaking driver= s need to be updated. Thanks, - Haiyang