Re: [RFC PATCH 0/2] Use high-order folios in mmap sync RA

Stepanov Anatoly
2026-04-15T13:33:52Z

On 4/15/2026 4:18 PM, Matthew Wilcox wrote:
> On Thu, Apr 16, 2026 at 03:28:51AM +0800, Anatoly Stepanov wrote:
>> When "fault around" is enabled, 0-order folios might significantly
>> slowdown filemap_map_pages().
>
> There's a lot of "might" in this patchset. I'd like to know that there
> is a real workload that benefits from this, and if so by how much.
>
Actually, no real workload at the moment.
The intention is to highlight the filemap_map_pages issue,
i found it during my experiments with the page cache.

> You raise an interesting point that faultaround may be slow, and maybe
> we should start out with 0 faultaround until we've determined (somehow)
> that faultaround would be beneficial for this particular mapping. Like
> we adjust the readahead window.
>
Sounds nice,
looks like, there should be kind of "virtual readahead" or smth like this.

BTW, for the benchmark i posted, if fault_around is disabled (4K)
then the throughput is even higher.

>> For example when async RA won't be able to start,
>> we might end up with a large mmap'ed file with 0-orders.
>
> That is a feature, not a bug. If access is random, then we don't want
> to do any async readahead because we don't know where the next access
> will be. We just end up occupying large chunks of memory with
> never-used data.
>
>
Yes, i understand the logic behind this, what i mean is that it can actually happen.

--
Anatoly Stepanov, Huawei