On 10/9/17 10:26 AM, Michal Hocko wrote:
On Tue 10-10-17 00:43:31, Yang Shi wrote:
On 10/8/17 11:48 PM, Michal Hocko wrote:
On Sun 08-10-17 15:56:51, Kirill A. Shutemov wrote:
On Sat, Oct 07, 2017 at 04:22:10AM +0800, Yang Shi wrote:
When passing "huge=always" option for mounting tmpfs, THP is supposed to
be allocated all the time when it can fit, but when the available space is
smaller than the size of THP (2MB on x86), shmem fault handler still tries
to allocate huge page every time, then fallback to regular 4K page
allocation, i.e.:
# mount -t tmpfs -o huge,size=3000k tmpfs /tmp
# dd if=/dev/zero of=/tmp/test bs=1k count=2048
# dd if=/dev/zero of=/tmp/test1 bs=1k count=2048
The last dd command will handle 952 times page fault handler, then exit
with -ENOSPC.
Rounding up tmpfs size to THP size in order to use THP with "always"
more efficiently. And, it will not wast too much memory (just allocate
511 extra pages in worst case).
Hm. I don't think it's good idea to silently increase size of fs.
Agreed!
Maybe better just refuse to mount with huge=always for too small fs?
We cannot we simply have the remaining page !THP? What is the actual
ups s@We@Why@
problem?
The remaining pages can be !THP, it will fall back to regular 4k pages when
the available space is less than THP size.
I just wonder it sounds not make sense to *not* mount tmpfs with THP size
alignment when "huge=always" is passed.
yes failure seems overly excessive reaction to me.
I guess someone would like to assume all allocation in tmpfs with
"huge=always" should be THP.
Nobody can assume that because THP pages can be broken up at any point
in time. We have hugetlb to provide a guarantee
But, they might not be fully aware of in some
corner cases THP might be not used, for example, the remaining space is less
then THP size, then some unexpected performance degrade might be perceived.
So, why not we do the mount correctly at the first place. It could be
delegated to the administrator, but it should be better to give some hint
from kernel side.
Because we are not trying to be more clever than the user. I still do
not see what is the actual problem you are trying to fix to be honest.
Just try to provide a warning or hint to the users that it'd better to
mount tmpfs with THP size aligned when "huge=always" is passed to avoid
some unexpected performance degrade.
Resizing or failure to mount might be overkill, documenting it might be
good enough.
Thanks,
Yang