On 19.11.25 15:37, David Hildenbrand (Red Hat) wrote:
Given folio_test_swapcache() might have false positives, I assume we'd need a
folio_test_swapbacked() && folio_test_swapcache(folio)
To detect large large shmem folios in the swapcache in all cases here.
Something like the following would hopefully do:
diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 2f2a521e5d683..57aab66bedbea 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -3515,6 +3515,13 @@ static int __split_unmapped_folio(struct folio *folio, int new_order, return ret; } +static bool folio_test_shmem_swapcache(struct folio *folio) +{
VM_WARN_ON_ONCE_FOLIO(folio_test_anon(folio), folio);/* These folios do not have folio->mapping set. */return folio_test_swapbacked(folio) && folio_test_swapcache(folio);+}
- bool non_uniform_split_supported(struct folio *folio, unsigned int new_order, bool warns) {
@@ -3524,6 +3531,9 @@ bool non_uniform_split_supported(struct folio *folio, unsigned int new_order, "Cannot split to order-1 folio"); if (new_order == 1) return false;
} else if (folio_test_shmem_swapcache(folio)) {/* TODO: support shmem folios that are in the swapcache. */return false;With this, truncated shmem returns -EINVALID instead of -EBUSY now. Can s390_wiggle_split_folio() such folios?
[noting that s390_wiggle_split_folio() was just one caller where I new the return value differs. I suspect there might be more.]
I am still not clear on that one.
s390x obtains the folio while walking the page tables. In case it gets -EBUSY it simply retries to obtain the folio from the page tables.
So assuming there was concurrent truncation and we returned -EBUSY, it would just retry walking the page tables (trigger a fault to map a folio) and retry with that one.
I would assume that the shmem folio in the swapcache could never have worked before, and that there is no way to make progress really.
In other words: do we know how we can end up with a shmem folio that is in the swapcache and does not have folio->mapping set?
Could that think still be mapped into the page tables? (I hope not, but right now I am confused how that can happen )
Ah, my memory comes back.
vmscan triggers shmem_writeout() after unmapping the folio and after making sure that there are no unexpected folio references.
shmem_writeout() will do the shmem_delete_from_page_cache() where we set folio->mapping = NULL.
So anything walking the page tables (like s390x) could never find it.
Such shmem folios really cannot get split right now until we either reclaimed them (-> freed) or until shmem_swapin_folio() re-obtained them from the swapcache to re-add them to the swapcache through shmem_add_to_page_cache().
So maybe we can just make our life easy and just keep returning -EBUSY for this scenario for the time being?
diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 2f2a521e5d683..5ce86882b2727 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -3619,6 +3619,16 @@ static int __folio_split(struct folio *folio, unsigned int new_order, if (folio != page_folio(split_at) || folio != page_folio(lock_at)) return -EINVAL;
+ /* + * Folios that just got truncated cannot get split. Signal to the + * caller that there was a race. + * + * TODO: this will also currently refuse shmem folios that are in + * the swapcache. + */ + if (!is_anon && !folio->mapping) + return -EBUSY; + if (new_order >= folio_order(folio)) return -EINVAL;
@@ -3659,17 +3669,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order, gfp_t gfp;
mapping = folio->mapping; - - /* Truncated ? */ - /* - * TODO: add support for large shmem folio in swap cache. - * When shmem is in swap cache, mapping is NULL and - * folio_test_swapcache() is true. - */ - if (!mapping) { - ret = -EBUSY; - goto out; - } + VM_WARN_ON_ONCE_FOLIO(!mapping, folio);
min_order = mapping_min_folio_order(folio->mapping); if (new_order < min_order) {