On Fri, Mar 27, 2026 at 11:00:26AM -0400, Zi Yan wrote: >On 27 Mar 2026, at 10:31, Lorenzo Stoakes (Oracle) wrote: > >> On Fri, Mar 27, 2026 at 10:26:53PM +0800, Baolin Wang wrote: >>> >>> >>> On 3/27/26 10:12 PM, Lorenzo Stoakes (Oracle) wrote: >>>> On Fri, Mar 27, 2026 at 09:45:03PM +0800, Baolin Wang wrote: >>>>> >>>>> >>>>> On 3/27/26 8:02 PM, Lorenzo Stoakes (Oracle) wrote: >>>>>> On Fri, Mar 27, 2026 at 05:44:49PM +0800, Baolin Wang wrote: >>>>>>> >>>>>>> >>>>>>> On 3/27/26 9:42 AM, Zi Yan wrote: >>>>>>>> collapse_file() requires FSes supporting large folio with at least >>>>>>>> PMD_ORDER, so replace the READ_ONLY_THP_FOR_FS check with that. shmem >>>>>>>> with >>>>>>>> huge option turned on also sets large folio order on mapping, so the >>>>>>>> check >>>>>>>> also applies to shmem. >>>>>>>> >>>>>>>> While at it, replace VM_BUG_ON with returning failure values. >>>>>>>> >>>>>>>> Signed-off-by: Zi Yan <[email protected]> >>>>>>>> --- >>>>>>>> mm/khugepaged.c | 7 +++++-- >>>>>>>> 1 file changed, 5 insertions(+), 2 deletions(-) >>>>>>>> >>>>>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>>>>>> index d06d84219e1b..45b12ffb1550 100644 >>>>>>>> --- a/mm/khugepaged.c >>>>>>>> +++ b/mm/khugepaged.c >>>>>>>> @@ -1899,8 +1899,11 @@ static enum scan_result collapse_file(struct >>>>>>>> mm_struct *mm, unsigned long addr, >>>>>>>> int nr_none = 0; >>>>>>>> bool is_shmem = shmem_file(file); >>>>>>>> - VM_BUG_ON(!IS_ENABLED(CONFIG_READ_ONLY_THP_FOR_FS) && >>>>>>>> !is_shmem); >>>>>>>> - VM_BUG_ON(start & (HPAGE_PMD_NR - 1)); >>>>>>>> + /* "huge" shmem sets mapping folio order and passes the check >>>>>>>> below */ >>>>>>>> + if (mapping_max_folio_order(mapping) < PMD_ORDER) >>>>>>>> + return SCAN_FAIL; >>>>>>> >>>>>>> This is not true for anonymous shmem, since its large order allocation >>>>>>> logic >>>>>>> is similar to anonymous memory. That means it will not call >>>>>>> mapping_set_large_folios() for anonymous shmem. >>>>>>> >>>>>>> So I think the check should be: >>>>>>> >>>>>>> if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER) >>>>>>> return SCAN_FAIL; >>>>>> >>>>>> Hmm but in shmem_init() we have: >>>>>> >>>>>> #ifdef CONFIG_TRANSPARENT_HUGEPAGE >>>>>> if (has_transparent_hugepage() && shmem_huge > SHMEM_HUGE_DENY) >>>>>> SHMEM_SB(shm_mnt->mnt_sb)->huge = shmem_huge; >>>>>> else >>>>>> shmem_huge = SHMEM_HUGE_NEVER; /* just in case it was patched */ >>>>>> >>>>>> /* >>>>>> * Default to setting PMD-sized THP to inherit the global setting and >>>>>> * disable all other multi-size THPs. >>>>>> */ >>>>>> if (!shmem_orders_configured) >>>>>> huge_shmem_orders_inherit = BIT(HPAGE_PMD_ORDER); >>>>>> #endif >>>>>> >>>>>> And shm_mnt->mnt_sb is the superblock used for anon shmem. Also >>>>>> shmem_enabled_store() updates that if necessary. >>>>>> >>>>>> So we're still fine right? >>>>>> >>>>>> __shmem_file_setup() (used for anon shmem) calls shmem_get_inode() -> >>>>>> __shmem_get_inode() which has: >>>>>> >>>>>> if (sbinfo->huge) >>>>>> mapping_set_large_folios(inode->i_mapping); >>>>>> >>>>>> Shared for both anon shmem and tmpfs-style shmem. >>>>>> >>>>>> So I think it's fine as-is. >>>>> >>>>> I'm afraid not. Sorry, I should have been clearer. >>>>> >>>>> First, anonymous shmem large order allocation is dynamically controlled >>>>> via >>>>> the global interface (/sys/kernel/mm/transparent_hugepage/shmem_enabled) >>>>> and >>>>> the mTHP interfaces >>>>> (/sys/kernel/mm/transparent_hugepage/hugepages-*kB/shmem_enabled). >>>>> >>>>> This means that during anonymous shmem initialization, these interfaces >>>>> might be set to 'never'. so it will not call mapping_set_large_folios() >>>>> because sbinfo->huge is 'SHMEM_HUGE_NEVER'. >>>>> >>>>> Even if shmem large order allocation is subsequently enabled via the >>>>> interfaces, __shmem_file_setup -> mapping_set_large_folios() is not called >>>>> again. >>>> >>>> I see your point, oh this is all a bit of a mess... >>>> >>>> It feels like entirely the wrong abstraction anyway, since at best you're >>>> getting a global 'is enabled'. >>>> >>>> I guess what happened before was we'd never call into this with ! r/o thp >>>> for fs >>>> && ! is_shmem. >>> >>> Right. >>> >>>> But now we are allowing it, but should STILL be gating on !is_shmem so >>>> yeah your >>>> suggestion is correct I think actualyl. >>>> >>>> I do hate: >>>> >>>> if (!is_shmem && mapping_max_folio_order(mapping) < PMD_ORDER) >>>> >>>> As a bit of code though. It's horrible. >>> >>> Indeed. >>> >>>> Let's abstract that... >>>> >>>> It'd be nice if we could find a way to clean things up in the lead up to >>>> changes >>>> in series like this instead of sticking with the mess, but I guess since it >>>> mostly removes stuff that's ok for now. >>> >>> I think this check can be removed from this patch. >>> >>> During the khugepaged's scan, it will call thp_vma_allowable_order() to >>> check if the VMA is allowed to collapse into a PMD. >>> >>> Specifically, within the call chain thp_vma_allowable_order() -> >>> __thp_vma_allowable_orders(), shmem is checked via >>> shmem_allowable_huge_orders(), while other FSes are checked via >>> file_thp_enabled(). > >But for madvise(MADV_COLLAPSE) case, IIRC, it ignores shmem huge config >and can perform collapse anyway. This means without !is_shmem the check >will break madvise(MADV_COLLAPSE). Let me know if I get it wrong, since
Right. That will break MADV_COLLAPSE, IIUC. For MADV_COLLAPSE on anonymous shmem, eligibility is determined by the TVA_FORCED_COLLAPSE path via shmem_allowable_huge_orders(), not by whether the inode mapping got mapping_set_large_folios() at creation time. Using mmap(MAP_SHARED | MAP_ANONYMOUS): - create time: shmem_enabled=never, hugepages-2048kB/shmem_enabled=never - collapse time: shmem_enabled=never, hugepages-2048kB/shmem_enabled=always With the !is_shmem guard, collapse succeeds. Without it, the same setup fails with -EINVAL. Thanks, Lance >I was in that TVA_FORCED_COLLAPSE email thread but does not remember >everything there. > > >> >> It sucks not to have an assert. Maybe in that case make it a >> VM_WARN_ON_ONCE(). > >Will do that as I replied to David already. > >> >> I hate that you're left tracing things back like that... >> >>> >>> For those other filesystems, Patch 5 has already added the following check, >>> which I think is sufficient to filter out those FSes that do not support >>> large folios: >>> >>> if (mapping_max_folio_order(inode->i_mapping) < PMD_ORDER) >>> return false; >> >> 2 < 5, we won't tolerate bisection hazards. >> >>> >>> >>>>> Anonymous shmem behaves similarly to anonymous pages: it is controlled by >>>>> the 'shmem_enabled' interfaces and uses shmem_allowable_huge_orders() to >>>>> check for allowed large orders, rather than relying on >>>>> mapping_max_folio_order(). >>>>> >>>>> The mapping_max_folio_order() is intended to control large page allocation >>>>> only for tmpfs mounts. Therefore, I find the current code confusing and >>>>> think it needs to be fixed: >>>>> >>>>> /* Don't consider 'deny' for emergencies and 'force' for testing */ >>>>> if (sb != shm_mnt->mnt_sb && sbinfo->huge) >>>>> mapping_set_large_folios(inode->i_mapping); >>>> > >Hi Baolin, > >Do you want to send a fix for this? > >Also I wonder how I can distinguish between anonymous shmem code and tmpfs >code. >I thought they are the same thing except that they have different user >interface, >but it seems that I was wrong. > > >Best Regards, >Yan, Zi > >
