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().
It sucks not to have an assert. Maybe in that case make it a VM_WARN_ON_ONCE(). 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); > > > > Cheers, Lorenzo > Cheers, Lorenzo
