On Thu, Jul 20, 2023 at 4:34 AM Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> wrote: > > On Thu, Jul 20, 2023 at 4:55 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > > > On Thu, Jul 20, 2023 at 12:18 AM Sergey Senozhatsky > > <senozhatsky@xxxxxxxxxxxx> wrote: > > > > > > On (23/07/13 13:20), Hyeonggon Yoo wrote: > > > > The purpose of this series is to define own memory descriptor for zsmalloc, > > > > instead of re-using various fields of struct page. This is a part of the > > > > effort to reduce the size of struct page to unsigned long and enable > > > > dynamic allocation of memory descriptors. > > > > > > > > While [1] outlines this ultimate objective, the current use of struct page > > > > is highly dependent on its definition, making it challenging to separately > > > > allocate memory descriptors. > > > > > > I glanced through the series and it all looks pretty straight forward to > > > me. I'll have a closer look. And we definitely need Minchan to ACK it. > > > > > > > Therefore, this series introduces new descriptor for zsmalloc, called > > > > zsdesc. It overlays struct page for now, but will eventually be allocated > > > > independently in the future. > > > > > > So I don't expect zsmalloc memory usage increase. On one hand for each > > > physical page that zspage consists of we will allocate zsdesc (extra bytes), > > > but at the same time struct page gets slimmer. So we should be even, or > > > am I wrong? > > > > Well, it depends. Here is my understanding (which may be completely wrong): > > > > The end goal would be to have an 8-byte memdesc for each order-0 page, > > and then allocate a specialized struct per-folio according to the use > > case. In this case, we would have a memdesc and a zsdesc for each > > order-0 page. If sizeof(zsdesc) is 64 bytes (on 64-bit), then it's a > > net loss. The savings only start kicking in with higher order folios. > > As of now, zsmalloc only uses order-0 pages as far as I can tell, so > > the usage would increase if I understand correctly. > > I partially agree with you that the point of memdesc stuff is > allocating a use-case specific > descriptor per folio. but I thought the primary gain from memdesc was > from anon and file pages > (where high order pages are more usable), rather than zsmalloc. > > And I believe enabling a memory descriptor per folio would be > impossible (or inefficient) > if zsmalloc and other subsystems are using struct page in the current > way (or please tell me I'm wrong?) > > So I expect the primary gain would be from high-order anon/file folios, > while this series is a prerequisite for them to work sanely. Right, I agree with that, sorry if I wasn't clear. I meant that generally speaking, we see gains from memdesc from higher order folios, so for zsmalloc specifically we probably won't see seeing any savings, and *might* see some extra usage (which I might be wrong about, see below). > > > It seems to me though the sizeof(zsdesc) is actually 56 bytes (on > > 64-bit), so sizeof(zsdesc) + sizeof(memdesc) would be equal to the > > current size of struct page. If that's true, then there is no loss, > > Yeah, zsdesc would be 56 bytes on 64 bit CPUs as memcg_data field is > not used in zsmalloc. > More fields in the current struct page might not be needed in the > future, although it's hard to say at the moment. > but it's not a loss. Is page->memcg_data something that we can drop? Aren't there code paths that will check page->memcg_data even for kernel pages (e.g. __folio_put() -> __folio_put_small() -> mem_cgroup_uncharge() ) ? > > > and there's potential gain if we start using higher order folios in > > zsmalloc in the future. > > AFAICS zsmalloc should work even when the system memory is fragmented, > so we may implement fallback allocation (as currently discussed in > large anon folios thread). Of course, any usage of higher order folios in zsmalloc must have a fallback logic, although it might be simpler for zsmalloc than anon folios. I agree that's off topic here. > > It might work, but IMHO the purpose of this series is to enable memdesc > for large anon/file folios, rather than seeing a large gain in zsmalloc itself. > (But even in zsmalloc, it's not a loss) > > > (That is of course unless we want to maintain cache line alignment for > > the zsdescs, then we might end up using 64 bytes anyway). > > we already don't require cache line alignment for struct page. the current > alignment requirement is due to SLUB's cmpxchg128 operation, not cache > line alignment. I thought we want struct page to be cache line aligned (to avoid having to fetch two cache lines for one struct page), but I can easily be wrong. > > I might be wrong in some aspects, so please tell me if I am. > And thank you and Sergey for taking a look at this! Thanks to you for doing the work! > -- > Hyeonggon
