Re: Possible memory leak in 6.17.7

["David Wang" <00107082@163.com>]

At 2025-12-16 20:18:11, "David Wang" <00107082@163.com> wrote:
>
>At 2025-12-16 19:55:27, "Mal Haak" <malcolm@haak.id.au> wrote:
>>On Tue, 16 Dec 2025 17:09:18 +1000
>>Mal Haak <malcolm@haak.id.au> wrote:
>>
>>> On Tue, 16 Dec 2025 15:00:43 +0800 (CST)
>>> "David Wang" <00107082@163.com> wrote:
>>> 
>>> > At 2025-12-16 09:26:47, "Mal Haak" <malcolm@haak.id.au> wrote:  
>>> > >On Mon, 15 Dec 2025 19:42:56 +0000
>>> > >Viacheslav Dubeyko <Slava.Dubeyko@ibm.com> wrote:
>>> > >    
>>> > >> Hi Mal,
>>> > >>     
>>> > ><SNIP>     
>>> > >> 
>>> > >> Thanks a lot for reporting the issue. Finally, I can see the
>>> > >> discussion in email list. :) Are you working on the patch with
>>> > >> the fix? Should we wait for the fix or I need to start the issue
>>> > >> reproduction and investigation? I am simply trying to avoid
>>> > >> patches collision and, also, I have multiple other issues for
>>> > >> the fix in CephFS kernel client. :)
>>> > >> 
>>> > >> Thanks,
>>> > >> Slava.    
>>> > >
>>> > >Hello,
>>> > >
>>> > >Unfortunately creating a patch is just outside my comfort zone,
>>> > >I've lived too long in Lustre land.    
>>> > 
>>> > Hi, just out of curiosity, have you narrowed down the caller of
>>> > __filemap_get_folio causing the memory problem? Or do you have
>>> > trouble applying the debug patch for memory allocation profiling?
>>> > 
>>> > David 
>>> >   
>>> Hi David,
>>> 
>>> I hadn't yet as I did test XFS and NFS to see if it replicated the
>>> behaviour and it did not. 
>>> 
>>> But actually this could speed things up considerably. I will do that
>>> now and see what I get.
>>> 
>>> Thanks
>>> 
>>> Mal
>>> 
>>I did just give it a blast. 
>>
>>Unfortunately it returned exactly what I expected, that is the calls
>>are all coming from netfs.
>>
>>Which makes sense for cephfs. 
>>
>># sort -g /proc/allocinfo|tail|numfmt --to=iec
>>         10M     2541 drivers/block/zram/zram_drv.c:1597 [zram]
>>func:zram_meta_alloc 12M     3001 mm/execmem.c:41 func:execmem_vmalloc 
>>         12M     3605 kernel/fork.c:311 func:alloc_thread_stack_node 
>>         16M      992 mm/slub.c:3061 func:alloc_slab_page 
>>         20M    35544 lib/xarray.c:378 func:xas_alloc 
>>         31M     7704 mm/memory.c:1192 func:folio_prealloc 
>>         69M    17562 mm/memory.c:1190 func:folio_prealloc 
>>        104M     8212 mm/slub.c:3059 func:alloc_slab_page 
>>        124M    30075 mm/readahead.c:189 func:ractl_alloc_folio 
>>        2.6G   661392 fs/netfs/buffered_read.c:635 [netfs]
>>func:netfs_write_begin 
>>
>>So, unfortunately it doesn't reveal the true source. But was worth a
>>shot! So thanks again
>
>Oh,  at least cephfs could be ruled out, right?
ehh...., I think I could be wrong about this.....

>
>CC netfs folks then. :)

>
>
>>
>>Mal
>>
>>
>>> > >
>>> > >I've have been trying to narrow down a consistent reproducer that's
>>> > >as fast as my production workload. (It crashes a 32GB VM in 2hrs)
>>> > >And I haven't got it quite as fast. I think the dd workload is too
>>> > >well behaved. 
>>> > >
>>> > >I can confirm the issue appeared in the major patch set that was
>>> > >applied as part of the 6.15 kernel. So during the more complete
>>> > >pages to folios switch and that nothing has changed in the bug
>>> > >behaviour since then. I did have a look at all the diffs from 6.14
>>> > >to 6.18 on addr.c and didn't see any changes post 6.15 that looked
>>> > >like they would impact the bug behavior. 
>>> > >
>>> > >Again, I'm not super familiar with the CephFS code but to hazard a
>>> > >guess, but I think that the web download workload triggers things
>>> > >faster suggests that unaligned writes might make things worse. But
>>> > >again, I'm not 100% sure. I can't find a reproducer as fast as
>>> > >downloading a dataset. Rsync of lots and lots of tiny files is a
>>> > >tad faster than the dd case.
>>> > >
>>> > >I did see some changes in ceph_check_page_before_write where the
>>> > >previous code unlocked pages and then continued where as the
>>> > >changed folio code just returns ENODATA and doesn't unlock
>>> > >anything with most of the rest of the logic unchanged. This might
>>> > >be perfectly fine, but in my, admittedly limited, reading of the
>>> > >code I couldn't figure out where anything that was locked prior to
>>> > >this being called would get unlocked like it did prior to the
>>> > >change. Again, I could be miles off here and one of the bulk
>>> > >reclaim/unlock passes that was added might be cleaning this up
>>> > >correctly or some other functional change might take care of this,
>>> > >but it looks to be potentially in the code path I'm excising and
>>> > >it has had some unlock logic changed. 
>>> > >
>>> > >I've spent most of my time trying to find a solid quick reproducer.
>>> > >Not that it takes long to start leaking folios, but I wanted
>>> > >something that aggressively triggered it so a small vm would oom
>>> > >quickly and when combined with crash_on_oom it could potentially be
>>> > >used for regression testing by way of "did vm crash?".
>>> > >
>>> > >I'm not sure if it will super help, but I'll provide what details I
>>> > >can about the actual workload that really sets it off. It's a
>>> > >python based tool for downloading datasets. Datasets are split
>>> > >into N chunks and the tool downloads them in parallel 100 at a
>>> > >time until all N chunks are down. The compressed dataset is then
>>> > >unpacked and reassembled for use with workloads. 
>>> > >
>>> > >This is replicating a common home folder usecase in HPC. CephFS is
>>> > >very attractive for home folders due to it's "NFS-like" utility and
>>> > >performance. And many tools use a similar method for fetching large
>>> > >datasets. Tools are frequently written in python or go. 
>>> > >
>>> > >None of my customers have hit this yet, not have any enterprise
>>> > >customers as none have moved to a new enough kernel yet due to slow
>>> > >upgrade cycles. Even Proxmox have only just started testing on a
>>> > >kernel version > 6.14. 
>>> > >
>>> > >I'm more than happy to help however I can with testing. I can run
>>> > >instrumented kernels or test patches or whatever you need. I am
>>> > >sorry I haven't been able to produce a super clean, fast reproducer
>>> > >(my test cluster at home is all spinners and only 500TB usable).
>>> > >But I figured I needed to get the word out asap as distros and soon
>>> > >customers are going to be moving past 6.12-6.14 kernels as the 5-7
>>> > >year update cycle marches on. Especially those wanting to take full
>>> > >advantage of CacheFS and encryption functionality. 
>>> > >
>>> > >Again thanks for looking at this and do reach out if I can help in
>>> > >anyway. I am in the ceph slack if it's faster to reach out that
>>> > >way.
>>> > >
>>> > >Regards
>>> > >
>>> > >Mal Haak    
>>>