On 05/09/2024 00:10, Barry Song wrote: > On Thu, Sep 5, 2024 at 9:30 AM Usama Arif <usamaarif642@xxxxxxxxx> wrote: >> >> >> >> On 03/09/2024 23:05, Yosry Ahmed wrote: >>> On Tue, Sep 3, 2024 at 2:36 PM Barry Song <21cnbao@xxxxxxxxx> wrote: >>>> >>>> On Wed, Sep 4, 2024 at 8:08 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: >>>>> >>>>> On Tue, 3 Sep 2024 11:38:37 -0700 Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: >>>>> >>>>>>> [ 39.157954] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000007 >>>>>>> [ 39.158288] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001 >>>>>>> [ 39.158634] R13: 0000000000002b9a R14: 0000000000000000 R15: 00007ffd619d5518 >>>>>>> [ 39.158998] </TASK> >>>>>>> [ 39.159226] ---[ end trace 0000000000000000 ]--- >>>>>>> >>>>>>> After reverting this or Usama's "mm: store zero pages to be swapped >>>>>>> out in a bitmap", the problem is gone. I think these two patches may >>>>>>> have some conflict that needs to be resolved. >>>>>> >>>>>> Yup. I saw this conflict coming and specifically asked for this >>>>>> warning to be added in Usama's patch to catch it [1]. It served its >>>>>> purpose. >>>>>> >>>>>> Usama's patch does not handle large folio swapin, because at the time >>>>>> it was written we didn't have it. We expected Usama's series to land >>>>>> sooner than this one, so the warning was to make sure that this series >>>>>> handles large folio swapin in the zeromap code. Now that they are both >>>>>> in mm-unstable, we are gonna have to figure this out. >>>>>> >>>>>> I suspect Usama's patches are closer to land so it's better to handle >>>>>> this in this series, but I will leave it up to Usama and >>>>>> Chuanhua/Barry to figure this out :) >>>> >>>> I believe handling this in swap-in might violate layer separation. >>>> `swap_read_folio()` should be a reliable API to call, regardless of >>>> whether `zeromap` is present. Therefore, the fix should likely be >>>> within `zeromap` but not this `swap-in`. I’ll take a look at this with >>>> Usama :-) >>> >>> I meant handling it within this series to avoid blocking Usama >>> patches, not within this code. Thanks for taking a look, I am sure you >>> and Usama will figure out the best way forward :) >> >> Hi Barry and Yosry, >> >> Is the best (and quickest) way forward to have a v8 of this with >> https://lore.kernel.org/all/20240904055522.2376-1-21cnbao@xxxxxxxxx/ >> as the first patch, and using swap_zeromap_entries_count in alloc_swap_folio >> in this support large folios swap-in patch? > > Yes, Usama. i can actually do a check: > > zeromap_cnt = swap_zeromap_entries_count(entry, nr); > > /* swap_read_folio() can handle inconsistent zeromap in multiple entries */ > if (zeromap_cnt > 0 && zeromap_cnt < nr) > try next order; > > On the other hand, if you read the code of zRAM, you will find zRAM has > exactly the same mechanism as zeromap but zRAM can even do more > by same_pages filled. since zRAM does the job in swapfile layer, there > is no this kind of consistency issue like zeromap. > > So I feel for zRAM case, we don't need zeromap at all as there are duplicated > efforts while I really appreciate your job which can benefit all swapfiles. > i mean, zRAM has the ability to check "zero"(and also non-zero but same > content). after zeromap checks zeromap, zRAM will check again: > Yes, so there is a reason for having the zeromap patches, which I have outlined in the coverletter. https://lore.kernel.org/all/20240627105730.3110705-1-usamaarif642@xxxxxxxxx/ There are usecases where zswap/zram might not be used in production. We can reduce I/O and flash wear in those cases by a large amount. Also running in Meta production, we found that the number of non-zero filled complete pages were less than 1%, so essentially its only the zero-filled pages that matter. I believe after zeromap, it might be a good idea to remove the page_same_filled check from zram code? Its not really a problem if its kept as well as I dont believe any zero-filled pages should reach zram_write_page? > static int zram_write_page(struct zram *zram, struct page *page, u32 index) > { > ... > > if (page_same_filled(mem, &element)) { > kunmap_local(mem); > /* Free memory associated with this sector now. */ > flags = ZRAM_SAME; > atomic64_inc(&zram->stats.same_pages); > goto out; > } > ... > } > > So it seems that zeromap might slightly impact my zRAM use case. I'm not > blaming you, just pointing out that there might be some overlap in effort > here :-) > >> >> Thanks, >> Usama > > Thanks > Barry
