Memory pages not released by the filesystem after a truncate

[Mulyadi Santosa]

On Wed, Jul 6, 2016 at 6:24 AM, Houssem Daoud <houssem.daoud at polymtl.ca>
wrote:

> Hi,
>
> My system experiencing problems with atomic memory allocations. Device
> drivers are not able to allocate contiguous memory regions due to a high
> fragmentation level.
>
> At the time of failure: /proc/meminfo shows the following information:
> MemTotal: 4021820 Kb
> MemFree: 121912 Kb
> Active: 1304396 Kb
> Inactive: 2377124 Kb
>
> Most of the memory is consumed by the LRU inactive list and only 121 Mb
> is available to the system.
> By using a tracer, I found that most of the pages in the inactive list
> are created by the ext4 journal during a truncate operation.
> The call stack of the allocation is:
> [
> __alloc_pages_nodemask
> alloc_pages_current
> __page_cache_alloc
> find_or_create_page
> __getblk
> jbd2_journal_get_descriptor_buffer
> jbd2_journal_commit_transaction
> kjournald2
> kthread
> ]
>
> The problem is easily reproducible using the following script:
> #!/bin/bash
> while true;
> do
> dd if=/dev/zero of=output.dat  bs=100M count=1
> done
>
> Is that a normal behavior ? I know that the philosophy of memory
> management in Linux is to use the available memory as much as possible,
> but what is the need of keeping truncated pages in the LRU if we know
> that they are not even accessible ?
>
> The problem of the inactive list growth occurs only with the journal
> mode of ext4, not with the write-back mode.
>
> A chart representing the utilization of memory during the test is
> provided in this link:
> http://secretaire.dorsal.polymtl.ca/~hdaoud/ext4_journal_meminfo.png
>
> Thanks,
> Houssem
>
>
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>

Hi

Trying to help here:
You said you wanna do atomic allocation. But then you said you want to
allocate around ~100 MB contiguous memory region.

IIRC, if you want to do atomic allocation, usually it can not be that big.
I am not sure how large, but surely not reaching 100 MB. For that size, I
think you should rely on vmalloc.

But, for clarification, maybe you should also post your full content of
 /proc/buddyinfo and /proc/meminfo


-- 
regards,

Mulyadi Santosa
Freelance Linux trainer and consultant

blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com
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