On 11/06/2024 11:11, Baolin Wang wrote: > Commit 19eaf44954df adds multi-size THP (mTHP) for anonymous pages, that > can allow THP to be configured through the sysfs interface located at > '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. > > However, the anonymous shmem will ignore the anonymous mTHP rule > configured through the sysfs interface, and can only use the PMD-mapped > THP, that is not reasonable. Users expect to apply the mTHP rule for > all anonymous pages, including the anonymous shmem, in order to enjoy > the benefits of mTHP. For example, lower latency than PMD-mapped THP, > smaller memory bloat than PMD-mapped THP, contiguous PTEs on ARM architecture > to reduce TLB miss etc. In addition, the mTHP interfaces can be extended > to support all shmem/tmpfs scenarios in the future, especially for the > shmem mmap() case. > > The primary strategy is similar to supporting anonymous mTHP. Introduce > a new interface '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled', > which can have almost the same values as the top-level > '/sys/kernel/mm/transparent_hugepage/shmem_enabled', with adding a new > additional "inherit" option and dropping the testing options 'force' and > 'deny'. By default all sizes will be set to "never" except PMD size, > which is set to "inherit". This ensures backward compatibility with the > anonymous shmem enabled of the top level, meanwhile also allows independent > control of anonymous shmem enabled for each mTHP. > > Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> [...] Hi Baolin, I'm currently trying to fix a bug where khugepaged is not started if only shmem is enabled for THP. See discussion at [1]. It's been broken like this forever. Assuming anon and shmem THP are initially both disabled: # echo never > /sys/kernel/mm/transparent_hugepage/shmem_enabled # echo never > /sys/kernel/mm/transparent_hugepage/enabled <khugepaged is stopped here> Then shemem THP is enabled: # echo always > /sys/kernel/mm/transparent_hugepage/shmem_enabled <khugepaged is not started, this is a bug> As part of investigating the fix, I stumbled upon this patch, which I remember reviewing an early version of but I've been out for a while and missed the latter versions. See below for comments and questions; the answers to which will help me figure out how to fix the above... [1] https://lore.kernel.org/linux-mm/20240702144617.2291480-1-ryan.roberts@xxxxxxx/ > > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE > +static unsigned long shmem_allowable_huge_orders(struct inode *inode, > + struct vm_area_struct *vma, pgoff_t index, > + bool global_huge) > +{ > + unsigned long mask = READ_ONCE(huge_shmem_orders_always); > + unsigned long within_size_orders = READ_ONCE(huge_shmem_orders_within_size); > + unsigned long vm_flags = vma->vm_flags; > + /* > + * Check all the (large) orders below HPAGE_PMD_ORDER + 1 that > + * are enabled for this vma. > + */ > + unsigned long orders = BIT(PMD_ORDER + 1) - 1; > + loff_t i_size; > + int order; > + > + if ((vm_flags & VM_NOHUGEPAGE) || > + test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags)) > + return 0; > + > + /* If the hardware/firmware marked hugepage support disabled. */ > + if (transparent_hugepage_flags & (1 << TRANSPARENT_HUGEPAGE_UNSUPPORTED)) > + return 0; > + > + /* > + * Following the 'deny' semantics of the top level, force the huge > + * option off from all mounts. > + */ > + if (shmem_huge == SHMEM_HUGE_DENY) > + return 0; I don't quite get this, I don't think its the desirable behaviour. This is saying that if the top-level shemem_enabled control is set to 'deny', then all mTHP sizes, regardless of their control's setting are disabled? The anon controls don't work like that; you can set the top-level control to anything you like, but its only consumed if the per-size controls are inheriting it. So for the deny case, wouldn't it be better to allow that as an option on all the per-size controls (and implicitly let it be inherrited from the top-level)? > + > + /* > + * Only allow inherit orders if the top-level value is 'force', which > + * means non-PMD sized THP can not override 'huge' mount option now. > + */ > + if (shmem_huge == SHMEM_HUGE_FORCE) > + return READ_ONCE(huge_shmem_orders_inherit); I vaguely recall that we originally discussed that trying to set 'force' on the top level control while any per-size controls were set to 'inherit' would be an error, and trying to set 'force' on any per-size control except the PMD-size would be an error? I don't really understand this logic. Shouldn't we just be looking at the per-size control settings (or the top-level control as a proxy for every per-size control that has 'inherit' set)? Then for tmpfs, which doesn't support non-PMD-sizes yet, we just always use the PMD-size control for decisions. I'm also really struggling with the concept of shmem_is_huge() existing along side shmem_allowable_huge_orders(). Surely this needs to all be refactored into shmem_allowable_huge_orders()? > + > + /* Allow mTHP that will be fully within i_size. */ > + order = highest_order(within_size_orders); > + while (within_size_orders) { > + index = round_up(index + 1, order); > + i_size = round_up(i_size_read(inode), PAGE_SIZE); > + if (i_size >> PAGE_SHIFT >= index) { > + mask |= within_size_orders; > + break; > + } > + > + order = next_order(&within_size_orders, order); > + } > + > + if (vm_flags & VM_HUGEPAGE) > + mask |= READ_ONCE(huge_shmem_orders_madvise); > + > + if (global_huge) Perhaps I've misunderstood global_huge, but I think its just the return value from shmem_is_huge()? But you're also using shmem_huge directly in this function. I find it all rather confusing. Sorry if all this was discussed and agreed in my absence! But I think this needs to be sorted in order to do the bug fix for khugepaged properly. Thanks, Ryan > + mask |= READ_ONCE(huge_shmem_orders_inherit); > + > + return orders & mask; > +} > + > +static unsigned long shmem_suitable_orders(struct inode *inode, struct vm_fault *vmf, > + struct address_space *mapping, pgoff_t index, > + unsigned long orders) > +{ > + struct vm_area_struct *vma = vmf->vma; > + unsigned long pages; > + int order; > + > + orders = thp_vma_suitable_orders(vma, vmf->address, orders); > + if (!orders) > + return 0; > + > + /* Find the highest order that can add into the page cache */ > + order = highest_order(orders); > + while (orders) { > + pages = 1UL << order; > + index = round_down(index, pages); > + if (!xa_find(&mapping->i_pages, &index, > + index + pages - 1, XA_PRESENT)) > + break; > + order = next_order(&orders, order); > + } > + > + return orders; > +} > +#else > +static unsigned long shmem_allowable_huge_orders(struct inode *inode, > + struct vm_area_struct *vma, pgoff_t index, > + bool global_huge) > +{ > + return 0; > +} > + > +static unsigned long shmem_suitable_orders(struct inode *inode, struct vm_fault *vmf, > + struct address_space *mapping, pgoff_t index, > + unsigned long orders) > +{ > + return 0; > +} > +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */ > + > static struct folio *shmem_alloc_folio(gfp_t gfp, int order, > struct shmem_inode_info *info, pgoff_t index) > { > @@ -1625,38 +1726,55 @@ static struct folio *shmem_alloc_folio(gfp_t gfp, int order, > return folio; > } > > -static struct folio *shmem_alloc_and_add_folio(gfp_t gfp, > - struct inode *inode, pgoff_t index, > - struct mm_struct *fault_mm, bool huge) > +static struct folio *shmem_alloc_and_add_folio(struct vm_fault *vmf, > + gfp_t gfp, struct inode *inode, pgoff_t index, > + struct mm_struct *fault_mm, unsigned long orders) > { > struct address_space *mapping = inode->i_mapping; > struct shmem_inode_info *info = SHMEM_I(inode); > - struct folio *folio; > + struct vm_area_struct *vma = vmf ? vmf->vma : NULL; > + unsigned long suitable_orders = 0; > + struct folio *folio = NULL; > long pages; > - int error; > + int error, order; > > if (!IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) > - huge = false; > + orders = 0; > > - if (huge) { > - pages = HPAGE_PMD_NR; > - index = round_down(index, HPAGE_PMD_NR); > + if (orders > 0) { > + if (vma && vma_is_anon_shmem(vma)) { > + suitable_orders = shmem_suitable_orders(inode, vmf, > + mapping, index, orders); > + } else if (orders & BIT(HPAGE_PMD_ORDER)) { > + pages = HPAGE_PMD_NR; > + suitable_orders = BIT(HPAGE_PMD_ORDER); > + index = round_down(index, HPAGE_PMD_NR); > > - /* > - * Check for conflict before waiting on a huge allocation. > - * Conflict might be that a huge page has just been allocated > - * and added to page cache by a racing thread, or that there > - * is already at least one small page in the huge extent. > - * Be careful to retry when appropriate, but not forever! > - * Elsewhere -EEXIST would be the right code, but not here. > - */ > - if (xa_find(&mapping->i_pages, &index, > - index + HPAGE_PMD_NR - 1, XA_PRESENT)) > - return ERR_PTR(-E2BIG); > + /* > + * Check for conflict before waiting on a huge allocation. > + * Conflict might be that a huge page has just been allocated > + * and added to page cache by a racing thread, or that there > + * is already at least one small page in the huge extent. > + * Be careful to retry when appropriate, but not forever! > + * Elsewhere -EEXIST would be the right code, but not here. > + */ > + if (xa_find(&mapping->i_pages, &index, > + index + HPAGE_PMD_NR - 1, XA_PRESENT)) > + return ERR_PTR(-E2BIG); > + } > > - folio = shmem_alloc_folio(gfp, HPAGE_PMD_ORDER, info, index); > - if (!folio && pages == HPAGE_PMD_NR) > - count_vm_event(THP_FILE_FALLBACK); > + order = highest_order(suitable_orders); > + while (suitable_orders) { > + pages = 1UL << order; > + index = round_down(index, pages); > + folio = shmem_alloc_folio(gfp, order, info, index); > + if (folio) > + goto allocated; > + > + if (pages == HPAGE_PMD_NR) > + count_vm_event(THP_FILE_FALLBACK); > + order = next_order(&suitable_orders, order); > + } > } else { > pages = 1; > folio = shmem_alloc_folio(gfp, 0, info, index); > @@ -1664,6 +1782,7 @@ static struct folio *shmem_alloc_and_add_folio(gfp_t gfp, > if (!folio) > return ERR_PTR(-ENOMEM); > > +allocated: > __folio_set_locked(folio); > __folio_set_swapbacked(folio); > > @@ -1958,7 +2077,8 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index, > struct mm_struct *fault_mm; > struct folio *folio; > int error; > - bool alloced; > + bool alloced, huge; > + unsigned long orders = 0; > > if (WARN_ON_ONCE(!shmem_mapping(inode->i_mapping))) > return -EINVAL; > @@ -2030,14 +2150,21 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index, > return 0; > } > > - if (shmem_is_huge(inode, index, false, fault_mm, > - vma ? vma->vm_flags : 0)) { > + huge = shmem_is_huge(inode, index, false, fault_mm, > + vma ? vma->vm_flags : 0); > + /* Find hugepage orders that are allowed for anonymous shmem. */ > + if (vma && vma_is_anon_shmem(vma)) > + orders = shmem_allowable_huge_orders(inode, vma, index, huge); > + else if (huge) > + orders = BIT(HPAGE_PMD_ORDER); > + > + if (orders > 0) { > gfp_t huge_gfp; > > huge_gfp = vma_thp_gfp_mask(vma); > huge_gfp = limit_gfp_mask(huge_gfp, gfp); > - folio = shmem_alloc_and_add_folio(huge_gfp, > - inode, index, fault_mm, true); > + folio = shmem_alloc_and_add_folio(vmf, huge_gfp, > + inode, index, fault_mm, orders); > if (!IS_ERR(folio)) { > if (folio_test_pmd_mappable(folio)) > count_vm_event(THP_FILE_ALLOC); > @@ -2047,7 +2174,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index, > goto repeat; > } > > - folio = shmem_alloc_and_add_folio(gfp, inode, index, fault_mm, false); > + folio = shmem_alloc_and_add_folio(vmf, gfp, inode, index, fault_mm, 0); > if (IS_ERR(folio)) { > error = PTR_ERR(folio); > if (error == -EEXIST) > @@ -2058,7 +2185,7 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index, > > alloced: > alloced = true; > - if (folio_test_pmd_mappable(folio) && > + if (folio_test_large(folio) && > DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE) < > folio_next_index(folio) - 1) { > struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
