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Date:   Tue, 20 Apr 2021 15:29:02 -0700
From:   Saravanan D <saravanand@fb.com>
To:     <axboe@kernel.dk>, <linux-block@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
CC:     <jkkm@fb.com>, <tj@kernel.org>, <kernel-team@fb.com>,
        Calvin Owens <calvinowens@fb.com>
Subject: Re: [PATCH] brd: expose number of allocated pages in debugfs
Message-ID: <20210420222902.scz3jvcirmpnjqnr@devvm1945.atn0.facebook.com>
References: <20210416211829.4002947-1-saravanand@fb.com>
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On Fri, Apr 16, 2021 at 02:18:29PM -0700, Saravanan D wrote:
> From: Calvin Owens <calvinowens@fb.com>
> 
> While the maximum size of each ramdisk is defined either
> as a module parameter, or compile time default, it's impossible
> to know how many pages have currently been allocated by each
> ram%d device, since they're allocated when used and never freed.
> 
> This patch creates a new directory at this location:
> 
> ?       /sys/kernel/debug/ramdisk_pages/
> 
> ...which will contain a file named "ram%d" for each instantiated
> ramdisk on the system. The file is read-only, and read() will
> output the number of pages currently held by that ramdisk.
> 

Justification : We lose track how much memory a ramdisk is using as
pages once used are simply recycled but never freed.

In instances where we exhaust the size of the ramdisk with a file that
exceeds it, encounter ENOSPC and delete the file for mitigation;
df would show decrease in used and increase in available blocks
but the since we have touched all pages, the memory footprint of the
ramdisk does not reflect the blocks used/available count

...
[root@localhost ~]# mkfs.ext2 /dev/ram15
mke2fs 1.45.6 (20-Mar-2020)
Creating filesystem with 4096 1k blocks and 1024 inodes
[root@localhost ~]# mount /dev/ram15 /mnt/ram15/

[root@localhost ~]# cat
/sys/kernel/debug/ramdisk_pages/ram15
58
[root@kerneltest008.06.prn3 ~]# df /dev/ram15
Filesystem     1K-blocks  Used Available Use% Mounted on
/dev/ram15          3963    31      3728   1% /mnt/ram15
[root@kerneltest008.06.prn3 ~]# dd if=/dev/urandom of=/mnt/ram15/test2
bs=1M count=5
dd: error writing '/mnt/ram15/test2': No space left on device
4+0 records in
3+0 records out
4005888 bytes (4.0 MB, 3.8 MiB) copied, 0.0446614 s, 89.7 MB/s
[root@kerneltest008.06.prn3 ~]# df /mnt/ram15/
Filesystem     1K-blocks  Used Available Use% Mounted on
/dev/ram15          3963  3960         0 100% /mnt/ram15
[root@kerneltest008.06.prn3 ~]# cat
/sys/kernel/debug/ramdisk_pages/ram15
1024
[root@kerneltest008.06.prn3 ~]# rm /mnt/ram15/test2
rm: remove regular file '/mnt/ram15/test2'? y
[root@kerneltest008.06.prn3 /var]# df /dev/ram15
Filesystem     1K-blocks  Used Available Use% Mounted on
/dev/ram15          3963    31      3728   1% /mnt/ram15

# Acutal memory footprint 
[root@kerneltest008.06.prn3 /var]# cat
/sys/kernel/debug/ramdisk_pages/ram15
1024
...

This debugfs counter will always reveal the accurate number of
permanently allocated pages to the ramdisk.