Re: [PATCH RESEND 0/8] hugetlb: add demote/split page functionality

From: Vlastimil Babka
Date: Fri Aug 27 2021 - 13:23:17 EST

On 8/25/21 00:08, Mike Kravetz wrote:
> Add Vlastimil and Hillf,
> Well, I set up a test environment on a larger system to get some
> numbers. My 'load' on the system was filling the page cache with
> clean pages. The thought is that these pages could easily be reclaimed.
> When trying to get numbers I hit a hugetlb page allocation stall where
> __alloc_pages(__GFP_RETRY_MAYFAIL, order 9) would stall forever (or at
> least an hour). It was very much like the symptoms addressed here:
> This was on 5.14.0-rc6-next-20210820.
> I'll do some more digging as this appears to be some dark corner case of
> reclaim and/or compaction. The 'good news' is that I can reproduce
> this.

Interesting, let's see if that's some kind of new regression.

>> And the second problem would benefit from some words to help us
>> understand how much real-world hurt this causes, and how frequently.
>> And let's understand what the userspace workarounds look like, etc.
> The stall above was from doing a simple 'free 1GB page' followed by
> 'allocate 512 MB pages' from userspace.

Is the allocation different in any way than the usual hugepage allocation
possible today?

> Getting out another version of this series will be delayed, as I think
> we need to address or understand this issue first.