On 4 Mar 2025, at 6:49, Hugh Dickins wrote: > On Wed, 26 Feb 2025, Zi Yan wrote: > >> This is a preparation patch, both added functions are not used yet. >> >> The added __split_unmapped_folio() is able to split a folio with its >> mapping removed in two manners: 1) uniform split (the existing way), and >> 2) buddy allocator like split. >> >> The added __split_folio_to_order() can split a folio into any lower order. >> For uniform split, __split_unmapped_folio() calls it once to split the >> given folio to the new order. For buddy allocator split, >> __split_unmapped_folio() calls it (folio_order - new_order) times and each >> time splits the folio containing the given page to one lower order. >> >> Signed-off-by: Zi Yan <ziy@xxxxxxxxxx> > > Sorry, I'm tired and don't really want to be writing this yet, but the > migrate "hotfix" has tipped my hand, and I need to get this out to you > before more days pass. Thank you for taking the time to test my patches. I really appreciate it. > > I'd been unable to complete even a single iteration of my "kernel builds > on huge tmpfs while swapping to SSD" testing during this current 6.14-rc > mm.git cycle (6.14-rc itself fine) - until the last week, when some > important fixes have come in, so I'm no longer getting I/O errors from > ext4-on-loop0-on-huge-tmpfs, and "Huh VM_FAULT_OOM leaked" warnings: good. This error should be related to the other patch I sent out on using xas_try_split() in shmem_large_entry_split(). Great to have you confirm it fixed some of the bugs. > > But I still can't get beyond a few iterations, a few minutes: there's > some corruption of user data, which usually manifests as a kernel build > failing because fixdep couldn't find some truncated-on-the-left pathname. It is likely that this patch might fix it (partially): https://lore.kernel.org/linux-mm/56EBE3B6-99EA-470E-B2B3-92C9C13032DF@xxxxxxxxxx/. Andrew has picked it yesterday. > > While it definitely bisected to your folio_split() series, it's quite > possible that you're merely exposing an existing bug to wider use. > > I've spent the last few days trying to track this down, but still not > succeeded: I'm still getting much the same corruption. But have been > folding in various fixes as I found them, even though they have not > solved the main problem at all. I'll return to trying to debug the > corruption "tomorrow". Thank you very much. This patchset has not got much review yet, your help is really appreciated. > > I think (might be wrong, I'm in a rush) my mods are all to this > "add two new (not yet used) functions for folio_split()" patch: > please merge them in if you agree. > > 1. From source inspection, it looks like a folio_set_order() was missed. > > 2. Why is swapcache only checked when folio_test_anon? I can see that > you've just copied that over from the old __split_huge_page(), but > it seems wrong to me here and there - I guess a relic from before > shmem could swap out a huge page. > > 3. Doing folio_next() inside the for(;;) is unsafe in those configs > which have to look up zone etc, I got an oops from the "new_folio" > loop; didn't hit an oops from the "release" loop but fixed that too. > > 4. While correcting anon versus mapping versus swap_cache, shortened > the lines by avoiding origin_folio->mapping and &release->page. All these fixes make sense to me. Thanks again for your effort. Hi Andrew, Do you mind folding Hugh’s fixes to this patch? Let me know if you prefer a V10. Thanks. > > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> > --- > mm/huge_memory.c | 39 ++++++++++++++++++++------------------- > 1 file changed, 20 insertions(+), 19 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 0e45937c0d91..9ce3906672b9 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -3612,7 +3612,9 @@ static void __split_folio_to_order(struct folio *folio, int new_order) > folio_xchg_last_cpupid(new_folio, folio_last_cpupid(folio)); > } > > - if (!new_order) > + if (new_order) > + folio_set_order(folio, new_order); > + else > ClearPageCompound(&folio->page); > } > > @@ -3682,7 +3684,9 @@ static int __split_unmapped_folio(struct folio *folio, int new_order, > int ret = 0; > bool stop_split = false; > > - if (folio_test_anon(folio) && folio_test_swapcache(folio)) { > + if (folio_test_swapcache(folio)) { > + VM_BUG_ON(mapping); > + > /* a swapcache folio can only be uniformly split to order-0 */ > if (!uniform_split || new_order != 0) > return -EINVAL; > @@ -3750,9 +3754,8 @@ static int __split_unmapped_folio(struct folio *folio, int new_order, > * is new_order, since the folio will be worked on in next > * iteration. > */ > - for (release = folio, next = folio_next(folio); > - release != end_folio; > - release = next, next = folio_next(next)) { > + for (release = folio; release != end_folio; release = next) { > + next = folio_next(release); > /* > * for buddy allocator like split, the folio containing > * page will be split next and should not be released, > @@ -3784,32 +3787,31 @@ static int __split_unmapped_folio(struct folio *folio, int new_order, > lru_add_page_tail(origin_folio, &release->page, > lruvec, list); > > - /* Some pages can be beyond EOF: drop them from page cache */ > + /* Some pages can be beyond EOF: drop them from cache */ > if (release->index >= end) { > - if (shmem_mapping(origin_folio->mapping)) > + if (shmem_mapping(mapping)) > nr_dropped += folio_nr_pages(release); > else if (folio_test_clear_dirty(release)) > folio_account_cleaned(release, > - inode_to_wb(origin_folio->mapping->host)); > + inode_to_wb(mapping->host)); > __filemap_remove_folio(release, NULL); > folio_put(release); > - } else if (!folio_test_anon(release)) { > - __xa_store(&origin_folio->mapping->i_pages, > - release->index, &release->page, 0); > + } 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->page, 0); > + release, 0); > } > } > } > > unlock_page_lruvec(lruvec); > > - if (folio_test_anon(origin_folio)) { > - if (folio_test_swapcache(origin_folio)) > - xa_unlock(&swap_cache->i_pages); > - } else > + 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 */ > @@ -3828,9 +3830,8 @@ static int __split_unmapped_folio(struct folio *folio, int new_order, > * For buddy allocator like split, the first after-split folio is left > * for caller to unlock. > */ > - for (new_folio = origin_folio, next = folio_next(origin_folio); > - new_folio != next_folio; > - new_folio = next, next = folio_next(next)) { > + 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; > > -- > 2.43.0 Best Regards, Yan, Zi
