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 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
