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Wed, 05 Nov 2025 17:03:34 -0800 (PST) Date: Wed, 5 Nov 2025 17:03:31 -0800 From: Calvin Owens To: Qu Wenruo Cc: linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, Chris Mason , David Sterba Subject: Re: [QUESTION] Order-4 allocation failures on reads with 256bit csums Message-ID: References: <20251105180054.511528-1-calvin@wbinvd.org> <0f137e3f-7b9b-4fbe-a8f7-2c90dad1a5fc@suse.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <0f137e3f-7b9b-4fbe-a8f7-2c90dad1a5fc@suse.com> On Thursday 11/06 at 07:31 +1030, Qu Wenruo wrote: > 在 2025/11/6 07:24, Qu Wenruo 写道: > > 在 2025/11/6 04:30, Calvin Owens 写道: > > > Hello all, > > > > > > I'm seeing order-4 allocation failures reading from btrfs filesystems > > > with blake2b/sha256 checksums, on a couple different machines. > > > > > > I don't think I'm doing anything interesting: in both cases they were > > > idle except for a single-threaded file reader doing buffered I/O. The > > > first one was an x86 QEMU VM, the second was a raspberry pi 4b (below): > > Another thing is, although the order 4 allocation is indeed large, it's not > that unreasonable large. > > The problem is still that we're requiring physically contiguous range which > greatly reduce the chance to get one. > > Another point contributing to this is the order 4, which is beyond the > PAGE_ALLOC_COSTLY_ORDER (3), thus no more retry is done thus can fail here. > > In fact for your aarch64 case, you can configure the kernel to use 64K page > size and in that case such allocation will only be one page thus will almost > never fail. > > This leads to my final question, what's the memory size of the RPI4 and your > qemu VM? > My guess is there is a very limited amount of memory (1GiB?), but still a > lot of large buffered IOs. > I guess enlarging the VM RAM size will hugely reduce the chance of memory > allocation failure. Thanks for looking Qu. Both have 8GB of RAM, here's the rest of the allocfail trace from the pi in case it's interesting: Mem-Info: active_anon:56 inactive_anon:3262 isolated_anon:0 active_file:421253 inactive_file:1487506 isolated_file:0 unevictable:0 dirty:51 writeback:0 slab_reclaimable:28851 slab_unreclaimable:13163 mapped:6531 shmem:13 pagetables:493 sec_pagetables:0 bounce:0 kernel_misc_reclaimable:0 free:39062 free_pcp:34 free_cma:15 Node 0 active_anon:224kB inactive_anon:13048kB active_file:1685012kB inactive_file:5950024kB unevictable:0kB isolated(anon):0kB isolated(file):0kB mapped:26124kB dirty:204kB writeback:0kB shmem:52kB shmem_thp:0kB shmem_pmdmapped:0kB anon_thp:0kB kernel_stack:3472kB pagetables:1972kB sec_pagetables:0kB all_unreclaimable? no Balloon:0kB DMA free:48336kB boost:12288kB min:19692kB low:21540kB high:23388kB reserved_highatomic:2048KB free_highatomic:2032KB active_anon:0kB inactive_anon:80kB active_file:274560kB inactive_file:545700kB unevictable:0kB writepending:0kB present:970752kB managed:880012kB mlocked:0kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:60kB lowmem_reserve[]: 0 3008 6962 6962 DMA32 free:53372kB boost:10240kB min:36236kB low:42732kB high:49228kB reserved_highatomic:2048KB free_highatomic:2016KB active_anon:32kB inactive_anon:124kB active_file:971292kB inactive_file:1993632kB unevictable:0kB writepending:24kB present:3080192kB managed:3080192kB mlocked:0kB bounce:0kB free_pcp:0kB local_pcp:0kB free_cma:0kB lowmem_reserve[]: 0 0 3954 3954 Normal free:54540kB boost:16384kB min:50560kB low:59104kB high:67648kB reserved_highatomic:2048KB free_highatomic:2032KB active_anon:192kB inactive_anon:12844kB active_file:439160kB inactive_file:3410608kB unevictable:0kB writepending:180kB present:4194304kB managed:4049512kB mlocked:0kB bounce:0kB free_pcp:136kB local_pcp:0kB free_cma:0kB lowmem_reserve[]: 0 0 0 0 DMA: 12*4kB (UM) 11*8kB (UM) 53*16kB (UMEH) 40*32kB (UMH) 18*64kB (UMH) 47*128kB (UMH) 34*256kB (UMH) 39*512kB (UMH) 10*1024kB (UMH) 0*2048kB 0*4096kB = 48344kB DMA32: 14*4kB (UME) 16*8kB (UME) 806*16kB (UMEH) 194*32kB (UMEH) 29*64kB (UEH) 50*128kB (UM) 34*256kB (UMEH) 22*512kB (UMH) 4*1024kB (UMH) 1*2048kB (M) 0*4096kB = 53656kB Normal: 3*4kB (U) 6*8kB (U) 10*16kB (UEH) 31*32kB (UMEH) 35*64kB (UMEH) 45*128kB (UMH) 13*256kB (MH) 74*512kB (MH) 4*1024kB (UH) 0*2048kB 0*4096kB = 54524kB Node 0 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=1048576kB Node 0 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=32768kB Node 0 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=2048kB Node 0 hugepages_total=0 hugepages_free=0 hugepages_surp=0 hugepages_size=64kB 1909321 total pagecache pages 532 pages in swap cache Free swap = 16764072kB Total swap = 16777212kB 2061312 pages RAM 0 pages HighMem/MovableOnly 58883 pages reserved 16384 pages cma reserved