The patch titled Subject: Revert "mm/cma.c: remove redundant cma_mutex lock" has been added to the -mm mm-hotfixes-unstable branch. Its filename is revert-mm-cmac-remove-redundant-cma_mutex-lock.patch

This patch should soon appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Dong Aisheng aisheng.dong@nxp.com Subject: Revert "mm/cma.c: remove redundant cma_mutex lock"

This reverts commit a4efc174b382fcdb which introduced a regression issue that when there're multiple processes allocating dma memory in parallel by calling dma_alloc_coherent(), it may fail sometimes as follows:

Error log: cma: cma_alloc: linux,cma: alloc failed, req-size: 148 pages, ret: -16 cma: number of available pages: 3@125+20@172+12@236+4@380+32@736+17@2287+23@2473+20@36076+99@40477+108@40852+44@41108+20@41196+108@41364+108@41620+ 108@42900+108@43156+483@44061+1763@45341+1440@47712+20@49324+20@49388+5076@49452+2304@55040+35@58141+20@58220+20@58284+ 7188@58348+84@66220+7276@66452+227@74525+6371@75549=> 33161 free of 81920 total pages

When issue happened, we saw there were still 33161 pages (129M) free CMA memory and a lot available free slots for 148 pages in CMA bitmap that we want to allocate.

When dumping memory info, we found that there was also ~342M normal memory, but only 1352K CMA memory left in buddy system while a lot of pageblocks were isolated.

Memory info log: Normal free:351096kB min:30000kB low:37500kB high:45000kB reserved_highatomic:0KB active_anon:98060kB inactive_anon:98948kB active_file:60864kB inactive_file:31776kB unevictable:0kB writepending:0kB present:1048576kB managed:1018328kB mlocked:0kB bounce:0kB free_pcp:220kB local_pcp:192kB free_cma:1352kB lowmem_reserve[]: 0 0 0 Normal: 78*4kB (UECI) 1772*8kB (UMECI) 1335*16kB (UMECI) 360*32kB (UMECI) 65*64kB (UMCI) 36*128kB (UMECI) 16*256kB (UMCI) 6*512kB (EI) 8*1024kB (UEI) 4*2048kB (MI) 8*4096kB (EI) 8*8192kB (UI) 3*16384kB (EI) 8*32768kB (M) = 489288kB

The root cause of this issue is that since commit a4efc174b382 ("mm/cma.c: remove redundant cma_mutex lock"), CMA supports concurrent memory allocation. It's possible that the memory range process A trying to alloc has already been isolated by the allocation of process B during memory migration.

The problem here is that the memory range isolated during one allocation by start_isolate_page_range() could be much bigger than the real size we want to alloc due to the range is aligned to MAX_ORDER_NR_PAGES.

Taking an ARMv7 platform with 1G memory as an example, when MAX_ORDER_NR_PAGES is big (e.g. 32M with max_order 14) and CMA memory is relatively small (e.g. 128M), there're only 4 MAX_ORDER slot, then it's very easy that all CMA memory may have already been isolated by other processes when one trying to allocate memory using dma_alloc_coherent(). Since current CMA code will only scan one time of whole available CMA memory, then dma_alloc_coherent() may easy fail due to contention with other processes.

This patch simply falls back to the original method that using cma_mutex to make alloc_contig_range() run sequentially to avoid the issue.

Link: https://lkml.kernel.org/r/20220509094551.3596244-1-aisheng.dong@nxp.com Link: https://lore.kernel.org/all/20220315144521.3810298-2-aisheng.dong@nxp.com/ Fixes: a4efc174b382 ("mm/cma.c: remove redundant cma_mutex lock") Signed-off-by: Dong Aisheng aisheng.dong@nxp.com Acked-by: Minchan Kim minchan@kernel.org Cc: Marek Szyprowski m.szyprowski@samsung.com Cc: Lecopzer Chen lecopzer.chen@mediatek.com Cc: David Hildenbrand david@redhat.com Cc: Vlastimil Babka vbabka@suse.cz Cc: stable@vger.kernel.org [5.11+] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/cma.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/mm/cma.c~revert-mm-cmac-remove-redundant-cma_mutex-lock +++ a/mm/cma.c @@ -37,6 +37,7 @@

struct cma cma_areas[MAX_CMA_AREAS]; unsigned cma_area_count; +static DEFINE_MUTEX(cma_mutex);

phys_addr_t cma_get_base(const struct cma *cma) { @@ -468,9 +469,10 @@ struct page *cma_alloc(struct cma *cma, spin_unlock_irq(&cma->lock);

pfn = cma->base_pfn + (bitmap_no << cma->order_per_bit); + mutex_lock(&cma_mutex); ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA, GFP_KERNEL | (no_warn ? __GFP_NOWARN : 0)); - + mutex_unlock(&cma_mutex); if (ret == 0) { page = pfn_to_page(pfn); break; _

Patches currently in -mm which might be from aisheng.dong@nxp.com are

revert-mm-cmac-remove-redundant-cma_mutex-lock.patch