Re: [PATCH v3 1/4] mm: swap: Remove CLUSTER_FLAG_HUGE from swap_cluster_info:flags

Barry Song
2024-03-04T05:43:10Z

On Mon, Mar 4, 2024 at 5:52 PM Barry Song <21cnbao@gmail.com> wrote:
>
> On Sat, Mar 2, 2024 at 6:08 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >
> > On 01/03/2024 16:44, Ryan Roberts wrote:
> > > On 01/03/2024 16:31, Matthew Wilcox wrote:
> > >> On Fri, Mar 01, 2024 at 04:27:32PM +0000, Ryan Roberts wrote:
> > >>> I've implemented the batching as David suggested, and I'm pretty confident it's
> > >>> correct. The only problem is that during testing I can't provoke the code to
> > >>> take the path. I've been pouring through the code but struggling to figure out
> > >>> under what situation you would expect the swap entry passed to
> > >>> free_swap_and_cache() to still have a cached folio? Does anyone have any idea?
> > >>>
> > >>> This is the original (unbatched) function, after my change, which caused David's
> > >>> concern that we would end up calling __try_to_reclaim_swap() far too much:
> > >>>
> > >>> int free_swap_and_cache(swp_entry_t entry)
> > >>> {
> > >>> struct swap_info_struct *p;
> > >>> unsigned char count;
> > >>>
> > >>> if (non_swap_entry(entry))
> > >>> return 1;
> > >>>
> > >>> p = _swap_info_get(entry);
> > >>> if (p) {
> > >>> count = __swap_entry_free(p, entry);
> > >>> if (count == SWAP_HAS_CACHE)
> > >>> __try_to_reclaim_swap(p, swp_offset(entry),
> > >>> TTRS_UNMAPPED | TTRS_FULL);
> > >>> }
> > >>> return p != NULL;
> > >>> }
> > >>>
> > >>> The trouble is, whenever its called, count is always 0, so
> > >>> __try_to_reclaim_swap() never gets called.
> > >>>
> > >>> My test case is allocating 1G anon memory, then doing madvise(MADV_PAGEOUT) over
> > >>> it. Then doing either a munmap() or madvise(MADV_FREE), both of which cause this
> > >>> function to be called for every PTE, but count is always 0 after
> > >>> __swap_entry_free() so __try_to_reclaim_swap() is never called. I've tried for
> > >>> order-0 as well as PTE- and PMD-mapped 2M THP.
> > >>
> > >> I think you have to page it back in again, then it will have an entry in
> > >> the swap cache. Maybe. I know little about anon memory ;-)
> > >
> > > Ahh, I was under the impression that the original folio is put into the swap
> > > cache at swap out, then (I guess) its removed once the IO is complete? I'm sure
> > > I'm miles out... what exactly is the lifecycle of a folio going through swap out?
> > >
> > > I guess I can try forking after swap out, then fault it back in in the child and
> > > exit. Then do the munmap in the parent. I guess that could force it? Thanks for
> > > the tip - I'll have a play.
> >
> > That has sort of solved it, the only problem now is that all the folios in the
> > swap cache are small (because I don't have Barry's large swap-in series). So
> > really I need to figure out how to avoid removing the folio from the cache in
> > the first place...
>
> I am quite sure we have a chance to hit a large swapcache even using zRAM -
> a sync swapfile and even during swap-out.
>
> I have a test case as below,
> 1. two threads to run MADV_PAGEOUT
> 2. two threads to read data being swapped-out
>
> in do_swap_page, from time to time, I can get a large swapcache.
>
> We have a short time window after add_to_swap() and before
> __removing_mapping() of
> vmscan, a large folio is still in swapcache.
>
> So Ryan, I guess you can trigger this by adding one more thread of
> MADV_DONTNEED to do zap_pte_range?

Ryan, I have modified my test case to have 4 threads:
1. MADV_PAGEOUT
2. MADV_DONTNEED
3. write data
4. read data

and git push the code here so that you can get it,
https://github.com/BarrySong666/swaptest/blob/main/swptest.c

I can reproduce the issue in zap_pte_range() in just a couple of minutes.

>
>
> >
> > >
> > >>
> > >> If that doesn't work, perhaps use tmpfs, and use some memory pressure to
> > >> force that to swap?
> > >>
> > >>> I'm guessing the swapcache was already reclaimed as part of MADV_PAGEOUT? I'm
> > >>> using a block ram device as my backing store - I think this does synchronous IO
> > >>> so perhaps if I have a real block device with async IO I might have more luck?
> > >>> Just a guess...
> > >>>
> > >>> Or perhaps this code path is a corner case? In which case, perhaps its not worth
> > >>> adding the batching optimization after all?
> > >>>
> > >>> Thanks,
> > >>> Ryan
> > >>>
> > >

Thanks
Barry