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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=DoXJefwmEF4Xv70LZdkYk6MtFwB06HynMMQiS9dxHAGyDBhDs6tIQxU6JqGlaxxT1 5l0KuSbVXUsmv7dZFj7pBdLci58Ix28p+xTpPcqqgCgREtAbIg718vw3jXbj/yM9py 6vibhorVUqWAuDsmXYGl7v3TKTbr8WA+aB2BJdKI= Message-ID: <46fbd241-4d64-409a-b9dc-77e778ca088e@arm.com> Date: Thu, 9 Apr 2026 15:40:38 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 5/8] mm/vmalloc: map contiguous pages in batches for vmap() if possible To: Barry Song Cc: linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, will@kernel.org, akpm@linux-foundation.org, urezki@gmail.com, linux-kernel@vger.kernel.org, anshuman.khandual@arm.com, ryan.roberts@arm.com, ajd@linux.ibm.com, rppt@kernel.org, david@kernel.org, Xueyuan.chen21@gmail.com References: <20260408025115.27368-1-baohua@kernel.org> <20260408025115.27368-6-baohua@kernel.org> Content-Language: en-US From: Dev Jain In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 6F061100007 X-Stat-Signature: bo535cifwdq59hc8scj98tx5wda8wkox X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1775729453-228618 X-HE-Meta: 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 83IQxig4 MPE8M1RlAK1l3QT2OI4U8qqR6nQ+H/Z95uDkCSN7RDkWWSKHYhA384nZpH8qwR0CXZ3aKWCDLzNARQBQQsnsFhcGETYAW6FxrJEqpk/yKkBWz0sJiNbcDxsdU2iikHRYh3VP+SPl1YzIFdfgziM0abMuRq5EKh7tyn8zHVwI6WP4I237sv65QWrQJx2+HXGhwmLt+DUrf+urSMjUw77FYylZmtlR3nBV0CfSvZWgzbVcqR8GZpcrlyI81e/mPqGSQChz9dKkcbt1uV0rqpa+YImffkdJTRSl33nxDtRUtFCwnzJSVTHEire4yxl91a/Q7AHPyz0NdPD1mf2Cv6Ey3ZgdVT/OgbHh0ZMesBSja5Y+n2gv4J5QfFmvwhJWqtvoz8Myr3OOh4uQ/Ejev8OUNHyr4KYo29cr+mEoH Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 09/04/26 3:24 am, Barry Song wrote: > On Wed, Apr 8, 2026 at 10:03 PM Dev Jain wrote: >> >> >> >> On 08/04/26 8:21 am, Barry Song (Xiaomi) wrote: >>> In many cases, the pages passed to vmap() may include high-order >>> pages allocated with __GFP_COMP flags. For example, the systemheap >>> often allocates pages in descending order: order 8, then 4, then 0. >>> Currently, vmap() iterates over every page individually—even pages >>> inside a high-order block are handled one by one. >>> >>> This patch detects high-order pages and maps them as a single >>> contiguous block whenever possible. >>> >>> An alternative would be to implement a new API, vmap_sg(), but that >>> change seems to be large in scope. >>> >>> Signed-off-by: Barry Song (Xiaomi) >>> --- >>> mm/vmalloc.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++++-- >>> 1 file changed, 49 insertions(+), 2 deletions(-) >>> >>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c >>> index eba436386929..e8dbfada42bc 100644 >>> --- a/mm/vmalloc.c >>> +++ b/mm/vmalloc.c >>> @@ -3529,6 +3529,53 @@ void vunmap(const void *addr) >>> } >>> EXPORT_SYMBOL(vunmap); >>> >>> +static inline int get_vmap_batch_order(struct page **pages, >>> + unsigned int max_steps, unsigned int idx) >>> +{ >>> + unsigned int nr_pages; >>> + >>> + if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMAP) || >>> + ioremap_max_page_shift == PAGE_SHIFT) >>> + return 0; >>> + >>> + nr_pages = compound_nr(pages[idx]); >>> + if (nr_pages == 1 || max_steps < nr_pages) >>> + return 0; >> >> This assumes that the page array passed to vmap() will have compound pages >> if it is a higher order allocation. >> >> See rb_alloc_aux_page(). It gets higher-order allocations without passing >> GFP_COMP. >> >> That is why my implementation does not assume anything about the property >> of the pages. > > If you’re asking about support for non-compound pages, I think > that’s fine. My current use case is dma-buf, where pages are > compound. I recall discussing this previously with David and > Uladzislau. > > If you’re working with non-compound pages, I’m happy to add > support in the next version. I’m also happy to reuse some of your > code and credit you as Co-developed-by if you’re willing. I actually > prefer your __vmap_huge() name over my > vmap_contig_pages_range(). > > Does that make sense to you? Yeah it will perhaps be better to have a fast-path detecting compound pages, and if not then checking contiguity. So sure please go ahead sharing some of my code and you can co-credit me. > >> >> Also it may be useful to do regression-testing for the common case of >> vmap() with a single page (assuming it is common, I don't know), in >> which case we may have to special case it. > > I agree, so I had Xueyuan test single pages and highlighted this > in the cover letter. There is no regression: "vmap() is 5.6× > faster when memory includes some order-8 pages, with no > regression observed for order-0 pages." > >> >> My implementation requires opting in with VM_ALLOW_HUGE_VMAP - I suspect >> you may run into problems if you make vmap() do huge-mappings as best-effort >> by default. I am guessing this because ... >> >> Drivers can operate on individual pages, so vmalloc() calls split_page() >> and then does the block/cont mappings. This same issue should be present >> with vmap() too? In which case if we are to do huge-mappings by default >> then we can do split_page() after detecting contiguous chunks. >> >> But ... that may create problems for the caller of vmap() - vmap now >> has the changed the properties of the pages. > > I don’t see this as a problem at all. Splitting pages does not > affect physical or virtual contiguity; it only changes the > contents of struct page objects, not the PTE/PMD mappings. > For ioremap, there isn’t even a struct page, yet the mappings > can still be huge. Okay so I was under the impression that *not* splitting the page will be problematic. But, vmalloc splits pages because the caller can operate on individual struct pages by vmalloc_to_page(). To the contrary, since the caller of vmap() decides what kind of pages to virtually-map, we don't have the problem I was raising. So I guess we are fine by making vmap do huge-mappings by default. > > Thanks > Barry