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List-Subscribe: List-Unsubscribe: On Mon, Jul 14, 2025 at 01:18:22PM -0400, Zi Yan wrote: > remap(), folio_ref_unfreeze(), lru_add_split_folio() are not relevant to > splitting unmapped folio operations. Move them out to the caller so that > __split_unmapped_folio() only handles unmapped folio splits. This makes > __split_unmapped_folio() reusable. Nit but maybe worth mentioning the various renames etc. > > Convert VM_BUG_ON(mapping) to use VM_WARN_ON_ONCE_FOLIO(). > > Signed-off-by: Zi Yan > Acked-by: Balbir Singh After a lot of staring, 2 difftastic's at once and exactly 0 coverity instances, I've convinced myself this looks right. I think you really should have split this up into smaller patches, as this is moving stuff around and changing stuff all at once with a lot of complexity and moving parts. However not going to make you do that, since you got acks and I don't want to hold this up. I have a few nits + queries below that need addressing however, see below. > --- > mm/huge_memory.c | 291 +++++++++++++++++++++++------------------------ > 1 file changed, 144 insertions(+), 147 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 3eb1c34be601..a7ee731f974f 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -3396,10 +3396,6 @@ static void __split_folio_to_order(struct folio *folio, int old_order, > * order - 1 to new_order). > * @split_at: in buddy allocator like split, the folio containing @split_at > * will be split until its order becomes @new_order. > - * @lock_at: the folio containing @lock_at is left locked for caller. > - * @list: the after split folios will be added to @list if it is not NULL, > - * otherwise to LRU lists. > - * @end: the end of the file @folio maps to. -1 if @folio is anonymous memory. > * @xas: xa_state pointing to folio->mapping->i_pages and locked by caller > * @mapping: @folio->mapping > * @uniform_split: if the split is uniform or not (buddy allocator like split) > @@ -3425,52 +3421,26 @@ static void __split_folio_to_order(struct folio *folio, int old_order, > * @page, which is split in next for loop. > * > * After splitting, the caller's folio reference will be transferred to the > - * folio containing @page. The other folios may be freed if they are not mapped. > - * > - * In terms of locking, after splitting, > - * 1. uniform split leaves @page (or the folio contains it) locked; > - * 2. buddy allocator like (non-uniform) split leaves @folio locked. Are these no longer relevant? Shouldn't we retain this, or move it elsewhere if appropriate? > - * > + * folio containing @page. The caller needs to unlock and/or free after-split > + * folios if necessary. > * > * For !uniform_split, when -ENOMEM is returned, the original folio might be > * split. The caller needs to check the input folio. > */ > static int __split_unmapped_folio(struct folio *folio, int new_order, > - struct page *split_at, struct page *lock_at, > - struct list_head *list, pgoff_t end, > - struct xa_state *xas, struct address_space *mapping, > - bool uniform_split) > + struct page *split_at, struct xa_state *xas, > + struct address_space *mapping, bool uniform_split) > { > - struct lruvec *lruvec; > - struct address_space *swap_cache = NULL; > - struct folio *origin_folio = folio; > - struct folio *next_folio = folio_next(folio); > - struct folio *new_folio; > struct folio *next; > int order = folio_order(folio); > int split_order; > int start_order = uniform_split ? new_order : order - 1; > - int nr_dropped = 0; > int ret = 0; > bool stop_split = false; > > - if (folio_test_swapcache(folio)) { > - VM_BUG_ON(mapping); Good to get rid of this. > - > - /* a swapcache folio can only be uniformly split to order-0 */ > - if (!uniform_split || new_order != 0) > - return -EINVAL; > - > - swap_cache = swap_address_space(folio->swap); > - xa_lock(&swap_cache->i_pages); > - } > - > if (folio_test_anon(folio)) > mod_mthp_stat(order, MTHP_STAT_NR_ANON, -1); > > - /* lock lru list/PageCompound, ref frozen by page_ref_freeze */ > - lruvec = folio_lruvec_lock(folio); > - > folio_clear_has_hwpoisoned(folio); > > /* > @@ -3480,9 +3450,9 @@ static int __split_unmapped_folio(struct folio *folio, int new_order, > for (split_order = start_order; > split_order >= new_order && !stop_split; > split_order--) { > - int old_order = folio_order(folio); > - struct folio *release; > struct folio *end_folio = folio_next(folio); > + int old_order = folio_order(folio); > + struct folio *new_folio; > > /* order-1 anonymous folio is not supported */ > if (folio_test_anon(folio) && split_order == 1) > @@ -3504,126 +3474,44 @@ static int __split_unmapped_folio(struct folio *folio, int new_order, > if (xas_error(xas)) { > ret = xas_error(xas); > stop_split = true; > - goto after_split; > } > } > } > > - folio_split_memcg_refs(folio, old_order, split_order); > - split_page_owner(&folio->page, old_order, split_order); > - pgalloc_tag_split(folio, old_order, split_order); > - > - __split_folio_to_order(folio, old_order, split_order); > + if (!stop_split) { > + folio_split_memcg_refs(folio, old_order, split_order); > + split_page_owner(&folio->page, old_order, split_order); > + pgalloc_tag_split(folio, old_order, split_order); > > -after_split: > + __split_folio_to_order(folio, old_order, split_order); > + } > /* > - * Iterate through after-split folios and perform related > - * operations. But in buddy allocator like split, the folio > + * Iterate through after-split folios and update folio stats. Good to spell out what the 'related operations' are :) Of course you're changing this so this loop does some and the other loop does the post-split rest. > + * But in buddy allocator like split, the folio > * containing the specified page is skipped until its order > * is new_order, since the folio will be worked on in next > * iteration. > */ > - for (release = folio; release != end_folio; release = next) { > - next = folio_next(release); > + for (new_folio = folio; new_folio != end_folio; new_folio = next) { > + next = folio_next(new_folio); > /* > - * for buddy allocator like split, the folio containing > - * page will be split next and should not be released, > - * until the folio's order is new_order or stop_split > - * is set to true by the above xas_split() failure. > + * for buddy allocator like split, new_folio containing > + * page could be split again, thus do not change stats > + * yet. Wait until new_folio's order is new_order or > + * stop_split is set to true by the above xas_split() > + * failure. > */ > - if (release == page_folio(split_at)) { > - folio = release; > + if (new_folio == page_folio(split_at)) { > + folio = new_folio; > if (split_order != new_order && !stop_split) > continue; > } > - if (folio_test_anon(release)) { > - mod_mthp_stat(folio_order(release), > - MTHP_STAT_NR_ANON, 1); > - } > - > - /* > - * origin_folio should be kept frozon until page cache > - * entries are updated with all the other after-split > - * folios to prevent others seeing stale page cache > - * entries. > - */ > - if (release == origin_folio) > - continue; > - > - folio_ref_unfreeze(release, 1 + > - ((mapping || swap_cache) ? > - folio_nr_pages(release) : 0)); > - > - lru_add_split_folio(origin_folio, release, lruvec, > - list); > - > - /* Some pages can be beyond EOF: drop them from cache */ > - if (release->index >= end) { > - if (shmem_mapping(mapping)) > - nr_dropped += folio_nr_pages(release); > - else if (folio_test_clear_dirty(release)) > - folio_account_cleaned(release, > - inode_to_wb(mapping->host)); > - __filemap_remove_folio(release, NULL); > - folio_put_refs(release, folio_nr_pages(release)); > - } else if (mapping) { > - __xa_store(&mapping->i_pages, > - release->index, release, 0); > - } else if (swap_cache) { > - __xa_store(&swap_cache->i_pages, > - swap_cache_index(release->swap), > - release, 0); > - } > + if (folio_test_anon(new_folio)) > + mod_mthp_stat(folio_order(new_folio), > + MTHP_STAT_NR_ANON, 1); > } > } > > - /* > - * Unfreeze origin_folio only after all page cache entries, which used > - * to point to it, have been updated with new folios. Otherwise, > - * a parallel folio_try_get() can grab origin_folio and its caller can > - * see stale page cache entries. > - */ > - folio_ref_unfreeze(origin_folio, 1 + > - ((mapping || swap_cache) ? folio_nr_pages(origin_folio) : 0)); > - > - unlock_page_lruvec(lruvec); > - > - if (swap_cache) > - xa_unlock(&swap_cache->i_pages); > - if (mapping) > - xa_unlock(&mapping->i_pages); > - > - /* Caller disabled irqs, so they are still disabled here */ > - local_irq_enable(); > - > - if (nr_dropped) > - shmem_uncharge(mapping->host, nr_dropped); > - > - remap_page(origin_folio, 1 << order, > - folio_test_anon(origin_folio) ? > - RMP_USE_SHARED_ZEROPAGE : 0); > - > - /* > - * At this point, folio should contain the specified page. > - * For uniform split, it is left for caller to unlock. > - * For buddy allocator like split, the first after-split folio is left > - * for caller to unlock. > - */ > - for (new_folio = origin_folio; new_folio != next_folio; new_folio = next) { > - next = folio_next(new_folio); > - if (new_folio == page_folio(lock_at)) > - continue; > - > - folio_unlock(new_folio); > - /* > - * Subpages may be freed if there wasn't any mapping > - * like if add_to_swap() is running on a lru page that > - * had its mapping zapped. And freeing these pages > - * requires taking the lru_lock so we do the put_page > - * of the tail pages after the split is complete. > - */ > - free_folio_and_swap_cache(new_folio); > - } > return ret; > } > > @@ -3706,10 +3594,13 @@ static int __folio_split(struct folio *folio, unsigned int new_order, > { > struct deferred_split *ds_queue = get_deferred_split_queue(folio); > XA_STATE(xas, &folio->mapping->i_pages, folio->index); > + struct folio *next_folio = folio_next(folio); > bool is_anon = folio_test_anon(folio); > struct address_space *mapping = NULL; > struct anon_vma *anon_vma = NULL; > int order = folio_order(folio); > + struct folio *new_folio, *next; > + int nr_shmem_dropped = 0; > int extra_pins, ret; > pgoff_t end; > bool is_hzp; There's some VM_BUG_ON_FOLIO()'s in the code: VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio); VM_BUG_ON_FOLIO(!folio_test_large(folio), folio); That should probably be VM_WARN_ON() or VM_WARN_ON_ONCE(), maybe worth changing here too? > @@ -3833,13 +3724,18 @@ static int __folio_split(struct folio *folio, unsigned int new_order, > */ > xas_lock(&xas); > xas_reset(&xas); > - if (xas_load(&xas) != folio) > + if (xas_load(&xas) != folio) { > + ret = -EAGAIN; It is beyond words that the original logic manually set ret == -EAGAIN... And this is the only place we 'goto fail'. Yikes this code is a horror show. > goto fail; > + } > } > > /* Prevent deferred_split_scan() touching ->_refcount */ > spin_lock(&ds_queue->split_queue_lock); > if (folio_ref_freeze(folio, 1 + extra_pins)) { > + struct address_space *swap_cache = NULL; > + struct lruvec *lruvec; > + > if (folio_order(folio) > 1 && > !list_empty(&folio->_deferred_list)) { > ds_queue->split_queue_len--; > @@ -3873,18 +3769,119 @@ static int __folio_split(struct folio *folio, unsigned int new_order, > } > } > > - ret = __split_unmapped_folio(folio, new_order, > - split_at, lock_at, list, end, &xas, mapping, > - uniform_split); > + if (folio_test_swapcache(folio)) { > + if (mapping) { > + VM_WARN_ON_ONCE_FOLIO(mapping, folio); > + ret = -EINVAL; > + goto fail; It's a new code path (in prod we'd just carry on, or in debug we would haven oops'd), but I think valid. I wonder if this is almost over-cautious, as this would require a non-anon folio to be in the swap-cache (since the is_anon path will set mapping NUL). But at the same time, probably worth keeping in at least for now. > + } > + > + /* > + * a swapcache folio can only be uniformly split to > + * order-0 > + */ > + if (!uniform_split || new_order != 0) { > + ret = -EINVAL; > + goto fail; > + } > + > + swap_cache = swap_address_space(folio->swap); > + xa_lock(&swap_cache->i_pages); > + } > + > + /* lock lru list/PageCompound, ref frozen by page_ref_freeze */ > + lruvec = folio_lruvec_lock(folio); > + > + ret = __split_unmapped_folio(folio, new_order, split_at, &xas, > + mapping, uniform_split); > + > + /* > + * Unfreeze after-split folios and put them back to the right > + * list. @folio should be kept frozon until page cache entries > + * are updated with all the other after-split folios to prevent > + * others seeing stale page cache entries. > + */ > + for (new_folio = folio_next(folio); new_folio != next_folio; > + new_folio = next) { Hm now we have 'next' and 'next_folio', this is quite confusing. Seems to me new_folio should be end_folio no, like the original? And maybe then rename next to next_folio? As it is kinda inconsistent that it isn't suffixed with _folio anyway. > + next = folio_next(new_folio); > + We're no longer doing what would here be new_folio == origin_folio (previously, release == origin_folio). Is this correct? Why do we no longer ned to do this? Is it because __split_unmapped_folio() will somehow take care of this in advance/render this meaningless? This definitely needs to be mentioned in the commit message. > + folio_ref_unfreeze( > + new_folio, > + 1 + ((mapping || swap_cache) ? > + folio_nr_pages(new_folio) : > + 0)); Again, be nice to separate this out, but I think in a follow-up not here. > + > + lru_add_split_folio(folio, new_folio, lruvec, list); > + > + /* Some pages can be beyond EOF: drop them from cache */ > + if (new_folio->index >= end) { > + if (shmem_mapping(mapping)) > + nr_shmem_dropped += folio_nr_pages(new_folio); > + else if (folio_test_clear_dirty(new_folio)) > + folio_account_cleaned( > + new_folio, > + inode_to_wb(mapping->host)); > + __filemap_remove_folio(new_folio, NULL); > + folio_put_refs(new_folio, > + folio_nr_pages(new_folio)); > + } else if (mapping) { > + __xa_store(&mapping->i_pages, new_folio->index, > + new_folio, 0); > + } else if (swap_cache) { > + __xa_store(&swap_cache->i_pages, > + swap_cache_index(new_folio->swap), > + new_folio, 0); > + } > + } > + /* > + * Unfreeze @folio only after all page cache entries, which > + * used to point to it, have been updated with new folios. > + * Otherwise, a parallel folio_try_get() can grab origin_folio > + * and its caller can see stale page cache entries. > + */ > + folio_ref_unfreeze(folio, 1 + > + ((mapping || swap_cache) ? folio_nr_pages(folio) : 0)); This line is horrid, probably one for a future series but this sort of calculation of what the number of refs should be post-freeze should clearly be separated out or at least made abundantly clear in an open-coded implementation. > + > + unlock_page_lruvec(lruvec); > + > + if (swap_cache) > + xa_unlock(&swap_cache->i_pages); > } else { > spin_unlock(&ds_queue->split_queue_lock); > -fail: > - if (mapping) > - xas_unlock(&xas); > - local_irq_enable(); > - remap_page(folio, folio_nr_pages(folio), 0); > ret = -EAGAIN; > } > +fail: > + if (mapping) > + xas_unlock(&xas); > + > + local_irq_enable(); > + > + if (nr_shmem_dropped) > + shmem_uncharge(mapping->host, nr_shmem_dropped); > + > + remap_page(folio, 1 << order, > + !ret && folio_test_anon(folio) ? RMP_USE_SHARED_ZEROPAGE : > + 0); I really don't like this !ret but here, this isn't very readable. Something like: int flags; ... if (!ret && folio_test_anon(folio)) flags = RMP_USE_SHARED_ZERO_PAGE; remap_page(folio, 1 << order, flags); Would be better. But really this is all screaming out to be separated into parts of course. But that's one for a follow-up series... > + > + /* > + * Unlock all after-split folios except the one containing @lock_at > + * page. If @folio is not split, it will be kept locked. > + */ > + for (new_folio = folio; new_folio != next_folio; new_folio = next) { > + next = folio_next(new_folio); > + if (new_folio == page_folio(lock_at)) > + continue; > + > + folio_unlock(new_folio); > + /* > + * Subpages may be freed if there wasn't any mapping > + * like if add_to_swap() is running on a lru page that > + * had its mapping zapped. And freeing these pages > + * requires taking the lru_lock so we do the put_page > + * of the tail pages after the split is complete. > + */ > + free_folio_and_swap_cache(new_folio); > + } > > out_unlock: > if (anon_vma) { > -- > 2.47.2 > Generally I see why you're not using origin_folio any more since you can just use folio everywhere, but I wonder if this makes things more confusing. On the other hand, this function is already hugely confusing so maybe not a big deal and can be addressed in follow ups...