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Message-ID: <62fad23f-e8dc-4fd5-a82f-6419376465b5@redhat.com>
Date: Fri, 29 Aug 2025 16:34:54 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: linux-kernel@...r.kernel.org, Alexandru Elisei
<alexandru.elisei@....com>, Alexander Potapenko <glider@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Brendan Jackman <jackmanb@...gle.com>, Christoph Lameter <cl@...two.org>,
Dennis Zhou <dennis@...nel.org>, Dmitry Vyukov <dvyukov@...gle.com>,
dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
iommu@...ts.linux.dev, io-uring@...r.kernel.org,
Jason Gunthorpe <jgg@...dia.com>, Jens Axboe <axboe@...nel.dk>,
Johannes Weiner <hannes@...xchg.org>, John Hubbard <jhubbard@...dia.com>,
kasan-dev@...glegroups.com, kvm@...r.kernel.org,
"Liam R. Howlett" <Liam.Howlett@...cle.com>,
Linus Torvalds <torvalds@...ux-foundation.org>, linux-arm-kernel@...s.com,
linux-arm-kernel@...ts.infradead.org, linux-crypto@...r.kernel.org,
linux-ide@...r.kernel.org, linux-kselftest@...r.kernel.org,
linux-mips@...r.kernel.org, linux-mmc@...r.kernel.org, linux-mm@...ck.org,
linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
linux-scsi@...r.kernel.org, Marco Elver <elver@...gle.com>,
Marek Szyprowski <m.szyprowski@...sung.com>, Michal Hocko <mhocko@...e.com>,
Mike Rapoport <rppt@...nel.org>, Muchun Song <muchun.song@...ux.dev>,
netdev@...r.kernel.org, Oscar Salvador <osalvador@...e.de>,
Peter Xu <peterx@...hat.com>, Robin Murphy <robin.murphy@....com>,
Suren Baghdasaryan <surenb@...gle.com>, Tejun Heo <tj@...nel.org>,
virtualization@...ts.linux.dev, Vlastimil Babka <vbabka@...e.cz>,
wireguard@...ts.zx2c4.com, x86@...nel.org, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH v1 21/36] mm/cma: refuse handing out non-contiguous page
ranges
On 28.08.25 19:28, Lorenzo Stoakes wrote:
> On Thu, Aug 28, 2025 at 12:01:25AM +0200, David Hildenbrand wrote:
>> Let's disallow handing out PFN ranges with non-contiguous pages, so we
>> can remove the nth-page usage in __cma_alloc(), and so any callers don't
>> have to worry about that either when wanting to blindly iterate pages.
>>
>> This is really only a problem in configs with SPARSEMEM but without
>> SPARSEMEM_VMEMMAP, and only when we would cross memory sections in some
>> cases.
>
> I'm guessing this is something that we don't need to worry about in
> reality?
That my theory yes.
>
>>
>> Will this cause harm? Probably not, because it's mostly 32bit that does
>> not support SPARSEMEM_VMEMMAP. If this ever becomes a problem we could
>> look into allocating the memmap for the memory sections spanned by a
>> single CMA region in one go from memblock.
>>
>> Reviewed-by: Alexandru Elisei <alexandru.elisei@....com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
>
> LGTM other than refactoring point below.
>
> CMA stuff looks fine afaict after staring at it for a while, on proviso
> that handing out ranges within the same section is always going to be the
> case.
>
> Anyway overall,
>
> LGTM, so:
>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
>
>> ---
>> include/linux/mm.h | 6 ++++++
>> mm/cma.c | 39 ++++++++++++++++++++++++---------------
>> mm/util.c | 33 +++++++++++++++++++++++++++++++++
>> 3 files changed, 63 insertions(+), 15 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index f6880e3225c5c..2ca1eb2db63ec 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -209,9 +209,15 @@ extern unsigned long sysctl_user_reserve_kbytes;
>> extern unsigned long sysctl_admin_reserve_kbytes;
>>
>> #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
>> +bool page_range_contiguous(const struct page *page, unsigned long nr_pages);
>> #define nth_page(page,n) pfn_to_page(page_to_pfn((page)) + (n))
>> #else
>> #define nth_page(page,n) ((page) + (n))
>> +static inline bool page_range_contiguous(const struct page *page,
>> + unsigned long nr_pages)
>> +{
>> + return true;
>> +}
>> #endif
>>
>> /* to align the pointer to the (next) page boundary */
>> diff --git a/mm/cma.c b/mm/cma.c
>> index e56ec64d0567e..813e6dc7b0954 100644
>> --- a/mm/cma.c
>> +++ b/mm/cma.c
>> @@ -780,10 +780,8 @@ static int cma_range_alloc(struct cma *cma, struct cma_memrange *cmr,
>> unsigned long count, unsigned int align,
>> struct page **pagep, gfp_t gfp)
>> {
>> - unsigned long mask, offset;
>> - unsigned long pfn = -1;
>> - unsigned long start = 0;
>> unsigned long bitmap_maxno, bitmap_no, bitmap_count;
>> + unsigned long start, pfn, mask, offset;
>> int ret = -EBUSY;
>> struct page *page = NULL;
>>
>> @@ -795,7 +793,7 @@ static int cma_range_alloc(struct cma *cma, struct cma_memrange *cmr,
>> if (bitmap_count > bitmap_maxno)
>> goto out;
>>
>> - for (;;) {
>> + for (start = 0; ; start = bitmap_no + mask + 1) {
>> spin_lock_irq(&cma->lock);
>> /*
>> * If the request is larger than the available number
>> @@ -812,6 +810,22 @@ static int cma_range_alloc(struct cma *cma, struct cma_memrange *cmr,
>> spin_unlock_irq(&cma->lock);
>> break;
>> }
>> +
>> + pfn = cmr->base_pfn + (bitmap_no << cma->order_per_bit);
>> + page = pfn_to_page(pfn);
>> +
>> + /*
>> + * Do not hand out page ranges that are not contiguous, so
>> + * callers can just iterate the pages without having to worry
>> + * about these corner cases.
>> + */
>> + if (!page_range_contiguous(page, count)) {
>> + spin_unlock_irq(&cma->lock);
>> + pr_warn_ratelimited("%s: %s: skipping incompatible area [0x%lx-0x%lx]",
>> + __func__, cma->name, pfn, pfn + count - 1);
>> + continue;
>> + }
>> +
>> bitmap_set(cmr->bitmap, bitmap_no, bitmap_count);
>> cma->available_count -= count;
>> /*
>> @@ -821,29 +835,24 @@ static int cma_range_alloc(struct cma *cma, struct cma_memrange *cmr,
>> */
>> spin_unlock_irq(&cma->lock);
>>
>> - pfn = cmr->base_pfn + (bitmap_no << cma->order_per_bit);
>> mutex_lock(&cma->alloc_mutex);
>> ret = alloc_contig_range(pfn, pfn + count, ACR_FLAGS_CMA, gfp);
>> mutex_unlock(&cma->alloc_mutex);
>> - if (ret == 0) {
>> - page = pfn_to_page(pfn);
>> + if (!ret)
>> break;
>> - }
>>
>> cma_clear_bitmap(cma, cmr, pfn, count);
>> if (ret != -EBUSY)
>> break;
>>
>> pr_debug("%s(): memory range at pfn 0x%lx %p is busy, retrying\n",
>> - __func__, pfn, pfn_to_page(pfn));
>> + __func__, pfn, page);
>>
>> - trace_cma_alloc_busy_retry(cma->name, pfn, pfn_to_page(pfn),
>> - count, align);
>> - /* try again with a bit different memory target */
>> - start = bitmap_no + mask + 1;
>> + trace_cma_alloc_busy_retry(cma->name, pfn, page, count, align);
>> }
>> out:
>> - *pagep = page;
>> + if (!ret)
>> + *pagep = page;
>> return ret;
>> }
>>
>> @@ -882,7 +891,7 @@ static struct page *__cma_alloc(struct cma *cma, unsigned long count,
>> */
>> if (page) {
>> for (i = 0; i < count; i++)
>> - page_kasan_tag_reset(nth_page(page, i));
>> + page_kasan_tag_reset(page + i);
>> }
>>
>> if (ret && !(gfp & __GFP_NOWARN)) {
>> diff --git a/mm/util.c b/mm/util.c
>> index d235b74f7aff7..0bf349b19b652 100644
>> --- a/mm/util.c
>> +++ b/mm/util.c
>> @@ -1280,4 +1280,37 @@ unsigned int folio_pte_batch(struct folio *folio, pte_t *ptep, pte_t pte,
>> {
>> return folio_pte_batch_flags(folio, NULL, ptep, &pte, max_nr, 0);
>> }
>> +
>> +#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
>> +/**
>> + * page_range_contiguous - test whether the page range is contiguous
>> + * @page: the start of the page range.
>> + * @nr_pages: the number of pages in the range.
>> + *
>> + * Test whether the page range is contiguous, such that they can be iterated
>> + * naively, corresponding to iterating a contiguous PFN range.
>> + *
>> + * This function should primarily only be used for debug checks, or when
>> + * working with page ranges that are not naturally contiguous (e.g., pages
>> + * within a folio are).
>> + *
>> + * Returns true if contiguous, otherwise false.
>> + */
>> +bool page_range_contiguous(const struct page *page, unsigned long nr_pages)
>> +{
>> + const unsigned long start_pfn = page_to_pfn(page);
>> + const unsigned long end_pfn = start_pfn + nr_pages;
>> + unsigned long pfn;
>> +
>> + /*
>> + * The memmap is allocated per memory section. We need to check
>> + * each involved memory section once.
>> + */
>> + for (pfn = ALIGN(start_pfn, PAGES_PER_SECTION);
>> + pfn < end_pfn; pfn += PAGES_PER_SECTION)
>> + if (unlikely(page + (pfn - start_pfn) != pfn_to_page(pfn)))
>> + return false;
>
> I find this pretty confusing, my test for this is how many times I have to read
> the code to understand what it's doing :)
>
> So we have something like:
>
> (pfn of page)
> start_pfn pfn = align UP
> | |
> v v
> | section |
> <----------------->
> pfn - start_pfn
>
> Then check page + (pfn - start_pfn) == pfn_to_page(pfn)
>
> And loop such that:
>
> (pfn of page)
> start_pfn pfn
> | |
> v v
> | section | section |
> <------------------------------------------>
> pfn - start_pfn
>
> Again check page + (pfn - start_pfn) == pfn_to_page(pfn)
>
> And so on.
>
> So the logic looks good, but it's just... that took me a hot second to
> parse :)
>
> I think a few simple fixups
>
> bool page_range_contiguous(const struct page *page, unsigned long nr_pages)
> {
> const unsigned long start_pfn = page_to_pfn(page);
> const unsigned long end_pfn = start_pfn + nr_pages;
> /* The PFN of the start of the next section. */
> unsigned long pfn = ALIGN(start_pfn, PAGES_PER_SECTION);
> /* The page we'd expected to see if the range were contiguous. */
> struct page *expected = page + (pfn - start_pfn);
>
> /*
> * The memmap is allocated per memory section. We need to check
> * each involved memory section once.
> */
> for (; pfn < end_pfn; pfn += PAGES_PER_SECTION, expected += PAGES_PER_SECTION)
> if (unlikely(expected != pfn_to_page(pfn)))
> return false;
> return true;
> }
>
Hm, I prefer my variant, especially where the pfn is calculated in the for loop. Likely a
matter of personal taste.
But I can see why skipping the first section might be a surprise when not
having the semantics of ALIGN() in the cache.
So I'll add the following on top:
diff --git a/mm/util.c b/mm/util.c
index 0bf349b19b652..fbdb73aaf35fe 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1303,8 +1303,10 @@ bool page_range_contiguous(const struct page *page, unsigned long nr_pages)
unsigned long pfn;
/*
- * The memmap is allocated per memory section. We need to check
- * each involved memory section once.
+ * The memmap is allocated per memory section, so no need to check
+ * within the first section. However, we need to check each other
+ * spanned memory section once, making sure the first page in a
+ * section could similarly be reached by just iterating pages.
*/
for (pfn = ALIGN(start_pfn, PAGES_PER_SECTION);
pfn < end_pfn; pfn += PAGES_PER_SECTION)
Thanks!
--
Cheers
David / dhildenb
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