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Date: Wed, 25 May 2022 23:48:01 +0200
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: Zi Yan <ziy@...dia.com>, Qian Cai <quic_qiancai@...cinc.com>,
Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>, linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org,
virtualization@...ts.linux-foundation.org,
Vlastimil Babka <vbabka@...e.cz>,
Mel Gorman <mgorman@...hsingularity.net>,
Eric Ren <renzhengeek@...il.com>,
Mike Rapoport <rppt@...nel.org>,
Oscar Salvador <osalvador@...e.de>,
Christophe Leroy <christophe.leroy@...roup.eu>
Subject: Re: [PATCH] mm: fix a potential infinite loop in
start_isolate_page_range().
On 24.05.2022 21:47, Zi Yan wrote:
> From: Zi Yan <ziy@...dia.com>
>
> In isolate_single_pageblock() called by start_isolate_page_range(),
> there are some pageblock isolation issues causing a potential
> infinite loop when isolating a page range. This is reported by Qian Cai.
>
> 1. the pageblock was isolated by just changing pageblock migratetype
> without checking unmovable pages. Calling set_migratetype_isolate() to
> isolate pageblock properly.
> 2. an off-by-one error caused migrating pages unnecessarily, since the page
> is not crossing pageblock boundary.
> 3. migrating a compound page across pageblock boundary then splitting the
> free page later has a small race window that the free page might be
> allocated again, so that the code will try again, causing an potential
> infinite loop. Temporarily set the to-be-migrated page's pageblock to
> MIGRATE_ISOLATE to prevent that and bail out early if no free page is
> found after page migration.
>
> An additional fix to split_free_page() aims to avoid crashing in
> __free_one_page(). When the free page is split at the specified
> split_pfn_offset, free_page_order should check both the first bit of
> free_page_pfn and the last bit of split_pfn_offset and use the smaller one.
> For example, if free_page_pfn=0x10000, split_pfn_offset=0xc000,
> free_page_order should first be 0x8000 then 0x4000, instead of 0x4000 then
> 0x8000, which the original algorithm did.
>
> Fixes: b2c9e2fbba ("mm: make alloc_contig_range work at pageblock granularity")
> Reported-by: Qian Cai <quic_qiancai@...cinc.com>
> Signed-off-by: Zi Yan <ziy@...dia.com>
This patch landed in linux next-20220525 as commit 29a8af92b874 ("mm:
fix a potential infinite loop in start_isolate_page_range()").
Unfortunately it breaks all CMA allocations done by the DMA-mapping
framework. I've observed this on ARM 32bit and 64bit. In the logs I only
see messages like:
cma: cma_alloc: linux,cma: alloc failed, req-size: 128 pages, ret: -16
I will try to analyze it a bit more tomorrow, but it looks that
isolation always fails.
> ---
> mm/page_alloc.c | 5 ++++-
> mm/page_isolation.c | 52 ++++++++++++++++++++++++++++++++++-----------
> 2 files changed, 44 insertions(+), 13 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 267599dd9706..6eec0211e0be 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1114,13 +1114,16 @@ void split_free_page(struct page *free_page,
> unsigned long flags;
> int free_page_order;
>
> + if (split_pfn_offset == 0)
> + return;
> +
> spin_lock_irqsave(&zone->lock, flags);
> del_page_from_free_list(free_page, zone, order);
> for (pfn = free_page_pfn;
> pfn < free_page_pfn + (1UL << order);) {
> int mt = get_pfnblock_migratetype(pfn_to_page(pfn), pfn);
>
> - free_page_order = ffs(split_pfn_offset) - 1;
> + free_page_order = min(pfn ? __ffs(pfn) : order, __fls(split_pfn_offset));
> __free_one_page(pfn_to_page(pfn), pfn, zone, free_page_order,
> mt, FPI_NONE);
> pfn += 1UL << free_page_order;
> diff --git a/mm/page_isolation.c b/mm/page_isolation.c
> index b3f074d1682e..c643c8420809 100644
> --- a/mm/page_isolation.c
> +++ b/mm/page_isolation.c
> @@ -283,6 +283,7 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
> * isolate_single_pageblock() -- tries to isolate a pageblock that might be
> * within a free or in-use page.
> * @boundary_pfn: pageblock-aligned pfn that a page might cross
> + * @flags: isolation flags
> * @gfp_flags: GFP flags used for migrating pages
> * @isolate_before: isolate the pageblock before the boundary_pfn
> *
> @@ -298,14 +299,15 @@ __first_valid_page(unsigned long pfn, unsigned long nr_pages)
> * either. The function handles this by splitting the free page or migrating
> * the in-use page then splitting the free page.
> */
> -static int isolate_single_pageblock(unsigned long boundary_pfn, gfp_t gfp_flags,
> - bool isolate_before)
> +static int isolate_single_pageblock(unsigned long boundary_pfn, int flags,
> + gfp_t gfp_flags, bool isolate_before)
> {
> unsigned char saved_mt;
> unsigned long start_pfn;
> unsigned long isolate_pageblock;
> unsigned long pfn;
> struct zone *zone;
> + int ret;
>
> VM_BUG_ON(!IS_ALIGNED(boundary_pfn, pageblock_nr_pages));
>
> @@ -325,7 +327,11 @@ static int isolate_single_pageblock(unsigned long boundary_pfn, gfp_t gfp_flags,
> zone->zone_start_pfn);
>
> saved_mt = get_pageblock_migratetype(pfn_to_page(isolate_pageblock));
> - set_pageblock_migratetype(pfn_to_page(isolate_pageblock), MIGRATE_ISOLATE);
> + ret = set_migratetype_isolate(pfn_to_page(isolate_pageblock), saved_mt, flags,
> + isolate_pageblock, isolate_pageblock + pageblock_nr_pages);
> +
> + if (ret)
> + return ret;
>
> /*
> * Bail out early when the to-be-isolated pageblock does not form
> @@ -374,7 +380,7 @@ static int isolate_single_pageblock(unsigned long boundary_pfn, gfp_t gfp_flags,
> struct page *head = compound_head(page);
> unsigned long head_pfn = page_to_pfn(head);
>
> - if (head_pfn + nr_pages < boundary_pfn) {
> + if (head_pfn + nr_pages <= boundary_pfn) {
> pfn = head_pfn + nr_pages;
> continue;
> }
> @@ -386,7 +392,8 @@ static int isolate_single_pageblock(unsigned long boundary_pfn, gfp_t gfp_flags,
> if (PageHuge(page) || PageLRU(page) || __PageMovable(page)) {
> int order;
> unsigned long outer_pfn;
> - int ret;
> + int page_mt = get_pageblock_migratetype(page);
> + bool isolate_page = !is_migrate_isolate_page(page);
> struct compact_control cc = {
> .nr_migratepages = 0,
> .order = -1,
> @@ -399,9 +406,31 @@ static int isolate_single_pageblock(unsigned long boundary_pfn, gfp_t gfp_flags,
> };
> INIT_LIST_HEAD(&cc.migratepages);
>
> + /*
> + * XXX: mark the page as MIGRATE_ISOLATE so that
> + * no one else can grab the freed page after migration.
> + * Ideally, the page should be freed as two separate
> + * pages to be added into separate migratetype free
> + * lists.
> + */
> + if (isolate_page) {
> + ret = set_migratetype_isolate(page, page_mt,
> + flags, head_pfn, head_pfn + nr_pages);
> + if (ret)
> + goto failed;
> + }
> +
> ret = __alloc_contig_migrate_range(&cc, head_pfn,
> head_pfn + nr_pages);
>
> + /*
> + * restore the page's migratetype so that it can
> + * be split into separate migratetype free lists
> + * later.
> + */
> + if (isolate_page)
> + unset_migratetype_isolate(page, page_mt);
> +
> if (ret)
> goto failed;
> /*
> @@ -417,10 +446,9 @@ static int isolate_single_pageblock(unsigned long boundary_pfn, gfp_t gfp_flags,
> order = 0;
> outer_pfn = pfn;
> while (!PageBuddy(pfn_to_page(outer_pfn))) {
> - if (++order >= MAX_ORDER) {
> - outer_pfn = pfn;
> - break;
> - }
> + /* stop if we cannot find the free page */
> + if (++order >= MAX_ORDER)
> + goto failed;
> outer_pfn &= ~0UL << order;
> }
> pfn = outer_pfn;
> @@ -435,7 +463,7 @@ static int isolate_single_pageblock(unsigned long boundary_pfn, gfp_t gfp_flags,
> return 0;
> failed:
> /* restore the original migratetype */
> - set_pageblock_migratetype(pfn_to_page(isolate_pageblock), saved_mt);
> + unset_migratetype_isolate(pfn_to_page(isolate_pageblock), saved_mt);
> return -EBUSY;
> }
>
> @@ -496,12 +524,12 @@ int start_isolate_page_range(unsigned long start_pfn, unsigned long end_pfn,
> int ret;
>
> /* isolate [isolate_start, isolate_start + pageblock_nr_pages) pageblock */
> - ret = isolate_single_pageblock(isolate_start, gfp_flags, false);
> + ret = isolate_single_pageblock(isolate_start, flags, gfp_flags, false);
> if (ret)
> return ret;
>
> /* isolate [isolate_end - pageblock_nr_pages, isolate_end) pageblock */
> - ret = isolate_single_pageblock(isolate_end, gfp_flags, true);
> + ret = isolate_single_pageblock(isolate_end, flags, gfp_flags, true);
> if (ret) {
> unset_migratetype_isolate(pfn_to_page(isolate_start), migratetype);
> return ret;
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
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