On Fri, Jul 05, 2024 at 10:59:02AM GMT, David Hildenbrand wrote: > On 05.07.24 10:45, Ryan Roberts wrote: > > On 05/07/2024 06:47, Baolin Wang wrote: > > > > > > > > > On 2024/7/5 03:49, Matthew Wilcox wrote: > > > > On Thu, Jul 04, 2024 at 09:19:10PM +0200, David Hildenbrand wrote: > > > > > On 04.07.24 21:03, David Hildenbrand wrote: > > > > > > > shmem has two uses: > > > > > > > > > > > > > > - MAP_ANONYMOUS | MAP_SHARED (this patch set) > > > > > > > - tmpfs > > > > > > > > > > > > > > For the second use case we don't want controls *at all*, we want the > > > > > > > same heiristics used for all other filesystems to apply to tmpfs. > > > > > > > > > > > > As discussed in the MM meeting, Hugh had a different opinion on that. > > > > > > > > > > FWIW, I just recalled that I wrote a quick summary: > > > > > > > > > > https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@xxxxxxxxxx > > > > > > > > > > I believe the meetings are recorded as well, but never looked at recordings. > > > > > > > > That's not what I understood Hugh to mean. To me, it seemed that Hugh > > > > was expressing an opinion on using shmem as shmem, not as using it as > > > > tmpfs. > > > > > > > > If I misunderstood Hugh, well, I still disagree. We should not have > > > > separate controls for this. tmpfs is just not that special. > > > > I wasn't at the meeting that's being referred to, but I thought we previously > > agreed that tmpfs *is* special because in some configurations its not backed by > > swap so is locked in ram? > > There are multiple things to that, like: > > * Machines only having limited/no swap configured > * tmpfs can be configured to never go to swap > * memfd/tmpfs files getting used purely for mmap(): there is no real > difference to MAP_ANON|MAP_SHARE besides the processes we share that > memory with. > > Especially when it comes to memory waste concerns and access behavior in > some cases, tmpfs behaved much more like anonymous memory. But there are for > sure other use cases where tmpfs is not that special. Having controls to select the allowable folio order allocations for tmpfs does not address any of these issues. The suggested filesystem approach [1] involves allocating orders in larger chunks, but always the same size you would allocate when using order-0 folios. So, it's a conservative approach. Using mTHP knobs in tmpfs would cause: * Over allocation when using mTHP and/ord THP under the 'always' flag. * Allocate in bigger chunks in a non optimal way, when not all mTHP and THP orders are enabled. * Operate in a similar manner as in [1] when all mTHP and THP orders are enabled and 'within_size' flag is used (assuming we use patch 11 from [1]). [1] Last 3 patches of these series: https://lore.kernel.org/all/20240515055719.32577-1-da.gomez@xxxxxxxxxxx/ My understanding of why mTHP was preferred is to raise awareness in user space and allow tmpfs mounts used at boot time to operate in 'safe' mode (no large folios). Does it make more sense to have a large folios enable flag to control order allocation as in [1], instead of every single order possible? > > My opinion is that we need to let people configure orders (if you feel like > it, configure all), but *select* the order to allocate based on readahead > information -- in contrast to anonymous memory where we start at the highest > order and don't have readahead information available. > > Maybe we need different "order allcoation" logic for read/write vs. fault, > not sure. I would suggest [1] the file size of the write for the write and fallocate paths. But when does make sense to use readahead information? Maybe when swap is involved? > > But I don't maintain that code, so I can only give stupid suggestions and > repeat what I understood from the meeting with Hugh and Kirill :) > > -- > Cheers, > > David / dhildenb >
