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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: o7zcts536zergb5kbnpjg4yfhsedqem9 X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 514851C001E X-HE-Tag: 1766059022-378231 X-HE-Meta: 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 qBK+tGz6 9BeS+Wksuiu462L2c38/lVLIBV4SglccjuH4BIZsMlPq/iPo/KfPeWHrIQE9Ad/a9+W+BzTHJhBF3AR2914NmUVFt5G1cvPAej/Ohzjy8yjn19LvMhEXLa1ev8vSrcApkYfQnis0FxcpbF7N/RBxjrvqY6U2oj4xgxaKvTFmuO1GY03Gc8WjQR85lYFA3H07eKdnKO04SXBBTc0B0h5qZMfhFQLWK1bRdXDaH21i/hqvO6r11XiNaVusZis8YdpdEUT0V/XvFGI+tZLMysTVQSK5b3JWNVho3ez+045mD+lrFOt8UQ/NxE48+xkmc8/ipIBC6plndQI/4x4ETYNgo9tYEmA== 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: + David, Lorenzo, Matthew Hoping someone might be able to explain to me how this all really works! :-| On 18/12/2025 11:53, Ryan Roberts wrote: > On 18/12/2025 04:55, Dev Jain wrote: >> >> On 17/12/25 8:50 pm, Ryan Roberts wrote: >>> On 17/12/2025 12:02, Uladzislau Rezki wrote: >>>>> On 16/12/2025 21:19, Uladzislau Rezki (Sony) wrote: >>>>>> Introduce a module parameter to enable or disable the large-order >>>>>> allocation path in vmalloc. High-order allocations are disabled by >>>>>> default so far, but users may explicitly enable them at runtime if >>>>>> desired. >>>>>> >>>>>> High-order pages allocated for vmalloc are immediately split into >>>>>> order-0 pages and later freed as order-0, which means they do not >>>>>> feed the per-CPU page caches. As a result, high-order attempts tend >>>>>> to bypass the PCP fastpath and fall back to the buddy allocator that >>>>>> can affect performance. >>>>>> >>>>>> However, when the PCP caches are empty, high-order allocations may >>>>>> show better performance characteristics especially for larger >>>>>> allocation requests. >>>>> I wonder if a better solution would be "allocate order-0 if available in pcp, >>>>> else try large order, else fallback to order-0" Could that provide the best of >>>>> all worlds without needing a configuration knob? >>>>> >>>> I am not sure, to me it looks like a bit odd. >>> Perhaps it would feel better if it was generalized to "first try allocation from >>> PCP list, highest to lowest order, then try allocation from the buddy, highest >>> to lowest order"? >>> >>>> Ideally it would be >>>> good just free it as high-order page and not order-0 peaces. >>> Yeah perhaps that's better. How about something like this (very lightly tested >>> and no performance results yet): >>> >>> (And I should admit I'm not 100% sure it is safe to call free_frozen_pages() >>> with a contiguous run of order-0 pages, but I'm not seeing any warnings or >>> memory leaks when running mm selftests...) >> >> Wow I wasn't aware that we can do this. I see that free_hotplug_page_range() in >> arm64/mmu.c already does this - it computes order from size and passes it to >> __free_pages(). > > Hmm that looks dodgy to me. But I'm not sure I actually understand what is going > on... > > Prior to looking at this yesterday, my understanding was this: At the struct > page level, you can either allocate compond or non-compound. order-0 is > non-compound by definition. A high-order non-compound page is just a contiguous > set of order-0 pages, each with individual reference counts and other meta data. > A compound page is one where all the pages are tied together and managed as one > - the meta data is stored in the head page and all the tail pages point to the > head (this concept is wrapped by struct folio). > > But after looking through the comments in page_alloc.c, it would seem that a > non-compound high-order page is NOT just a set of order-0 pages, but they still > share some meta data, including a shared refcount?? alloc_pages() will return > one of these things, and __free_pages() requires the exact same unit to be > provided to it. > > vmalloc calls alloc_pages() to get a non-compound high-order page, then calls > split_page() to convert to a set of order-0 pages. See this comment: > > /* > * split_page takes a non-compound higher-order page, and splits it into > * n (1< * Each sub-page must be freed individually. > * > * Note: this is probably too low level an operation for use in drivers. > * Please consult with lkml before using this in your driver. > */ > void split_page(struct page *page, unsigned int order) > > So just passing all the order-0 pages directly to __free_pages() in one go is > definitely not the right thing to do ("Each sub-page must be freed > individually"). They may have different reference counts so you can only > actually free the ones that go to zero surely? > > But it looked to me like free_frozen_pages() just wants a naturally aligned > power-of-2 number of pages to free, so my patch below is decrementing the > refcount on each struct page and accumulating the ones where the refcounts goto > zero into suitable blocks for free_frozen_pages(). > > So I *think* my patch is correct, but I'm not totally sure. > > Then we have the ___free_pages(), which I find very difficult to understand: > > static void ___free_pages(struct page *page, unsigned int order, > fpi_t fpi_flags) > { > /* get PageHead before we drop reference */ > int head = PageHead(page); > /* get alloc tag in case the page is released by others */ > struct alloc_tag *tag = pgalloc_tag_get(page); > > if (put_page_testzero(page)) > __free_frozen_pages(page, order, fpi_flags); > > We only test the refcount for the first page, then free all the pages. So that > implies that non-compound high-order pages share a single refcount? Or we just > ignore the refcount of all the other pages in a non-compound high-order page? > > else if (!head) { > > What? If the first page still has references but but it's a non-compond > high-order page (i.e. no head page) then we free all the trailing sub-pages > without caring about their references? > > pgalloc_tag_sub_pages(tag, (1 << order) - 1); > while (order-- > 0) { > /* > * The "tail" pages of this non-compound high-order > * page will have no code tags, so to avoid warnings > * mark them as empty. > */ > clear_page_tag_ref(page + (1 << order)); > __free_frozen_pages(page + (1 << order), order, > fpi_flags); > } > } > } > > For the arm64 case that you point out, surely __free_pages() is the wrong thing > to call, because it's going to decrement the refcount. But we are freeing based > on their presence in the pagetable and we never took a reference in the first place. > > HELP! > >> >>> >>> ---8<--- >>> commit caa3e5eb5bfade81a32fa62d1a8924df1eb0f619 >>> Author: Ryan Roberts >>> Date: Wed Dec 17 15:11:08 2025 +0000 >>> >>> WIP >>> >>> Signed-off-by: Ryan Roberts >>> >>> diff --git a/include/linux/gfp.h b/include/linux/gfp.h >>> index b155929af5b1..d25f5b867e6b 100644 >>> --- a/include/linux/gfp.h >>> +++ b/include/linux/gfp.h >>> @@ -383,6 +383,8 @@ extern void __free_pages(struct page *page, unsigned int order); >>> extern void free_pages_nolock(struct page *page, unsigned int order); >>> extern void free_pages(unsigned long addr, unsigned int order); >>> >>> +void free_pages_bulk(struct page *page, int nr_pages); >>> + >>> #define __free_page(page) __free_pages((page), 0) >>> #define free_page(addr) free_pages((addr), 0) >>> >>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >>> index 822e05f1a964..5f11224cf353 100644 >>> --- a/mm/page_alloc.c >>> +++ b/mm/page_alloc.c >>> @@ -5304,6 +5304,48 @@ static void ___free_pages(struct page *page, unsigned int >>> order, >>> } >>> } >>> >>> +static void free_frozen_pages_bulk(struct page *page, int nr_pages) >>> +{ >>> + while (nr_pages) { >>> + unsigned int fit_order, align_order, order; >>> + unsigned long pfn; >>> + >>> + pfn = page_to_pfn(page); >>> + fit_order = ilog2(nr_pages); >>> + align_order = pfn ? __ffs(pfn) : fit_order; >>> + order = min3(fit_order, align_order, MAX_PAGE_ORDER); >>> + >>> + free_frozen_pages(page, order); >>> + >>> + page += 1U << order; >>> + nr_pages -= 1U << order; >>> + } >>> +} >>> + >>> +void free_pages_bulk(struct page *page, int nr_pages) >>> +{ >>> + struct page *start = NULL; >>> + bool can_free; >>> + int i; >>> + >>> + for (i = 0; i < nr_pages; i++, page++) { >>> + VM_BUG_ON_PAGE(PageHead(page), page); >>> + VM_BUG_ON_PAGE(PageTail(page), page); >>> + >>> + can_free = put_page_testzero(page); >>> + >>> + if (!can_free && start) { >>> + free_frozen_pages_bulk(start, page - start); >>> + start = NULL; >>> + } else if (can_free && !start) { >>> + start = page; >>> + } >>> + } >>> + >>> + if (start) >>> + free_frozen_pages_bulk(start, page - start); >>> +} >>> + >>> /** >>> * __free_pages - Free pages allocated with alloc_pages(). >>> * @page: The page pointer returned from alloc_pages(). >>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c >>> index ecbac900c35f..8f782bac1ece 100644 >>> --- a/mm/vmalloc.c >>> +++ b/mm/vmalloc.c >>> @@ -3429,7 +3429,8 @@ void vfree_atomic(const void *addr) >>> void vfree(const void *addr) >>> { >>> struct vm_struct *vm; >>> - int i; >>> + struct page *start; >>> + int i, nr; >>> >>> if (unlikely(in_interrupt())) { >>> vfree_atomic(addr); >>> @@ -3455,17 +3456,26 @@ void vfree(const void *addr) >>> /* All pages of vm should be charged to same memcg, so use first one. */ >>> if (vm->nr_pages && !(vm->flags & VM_MAP_PUT_PAGES)) >>> mod_memcg_page_state(vm->pages[0], MEMCG_VMALLOC, -vm->nr_pages); >>> - for (i = 0; i < vm->nr_pages; i++) { >>> + >>> + start = vm->pages[0]; >>> + BUG_ON(!start); >>> + nr = 1; >>> + for (i = 1; i < vm->nr_pages; i++) { >>> struct page *page = vm->pages[i]; >>> >>> BUG_ON(!page); >>> - /* >>> - * High-order allocs for huge vmallocs are split, so >>> - * can be freed as an array of order-0 allocations >>> - */ >>> - __free_page(page); >>> - cond_resched(); >>> + >>> + if (start + nr != page) { >>> + free_pages_bulk(start, nr); >>> + start = page; >>> + nr = 1; >>> + cond_resched(); >>> + } else { >>> + nr++; >>> + } >>> } >>> + free_pages_bulk(start, nr); >>> + >>> if (!(vm->flags & VM_MAP_PUT_PAGES)) >>> atomic_long_sub(vm->nr_pages, &nr_vmalloc_pages); >>> kvfree(vm->pages); >>> ---8<--- >>> >>>>>> Since the best strategy is workload-dependent, this patch adds a >>>>>> parameter letting users to choose whether vmalloc should try >>>>>> high-order allocations or stay strictly on the order-0 fastpath. >>>>>> >>>>>> Signed-off-by: Uladzislau Rezki (Sony) >>>>>> --- >>>>>> mm/vmalloc.c | 9 +++++++-- >>>>>> 1 file changed, 7 insertions(+), 2 deletions(-) >>>>>> >>>>>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c >>>>>> index d3a4725e15ca..f66543896b16 100644 >>>>>> --- a/mm/vmalloc.c >>>>>> +++ b/mm/vmalloc.c >>>>>> @@ -43,6 +43,7 @@ >>>>>> #include >>>>>> #include >>>>>> #include >>>>>> +#include >>>>>> >>>>>> #define CREATE_TRACE_POINTS >>>>>> #include >>>>>> @@ -3671,6 +3672,9 @@ vm_area_alloc_pages_large_order(gfp_t gfp, int nid, unsigned int order, >>>>>> return nr_allocated; >>>>>> } >>>>>> >>>>>> +static int attempt_larger_order_alloc; >>>>>> +module_param(attempt_larger_order_alloc, int, 0644); >>>>> Would this be better as a bool? Docs say that you can then specify 0/1, y/n or >>>>> Y/N as the value; that's probably more intuitive? >>>>> >>>>> nit: I'd favour a shorter name. Perhaps large_order_alloc? >>>>> >>>> Thanks! We can switch to bool and use shorter name for sure. >>>> >>>> -- >>>> Uladzislau Rezki >