Re: [RFC PATCH 4/5] mm: swap: fall back to order-0 after large swapin races

[David Hildenbrand (Arm)]

On 5/8/26 22:20, fujunjie wrote:
> swapin_folio() documents that a large folio insertion race returns NULL
> so the caller can fall back to order-0 swapin. do_swap_page() currently
> turns that NULL into VM_FAULT_OOM if the PTE is unchanged, which is
> harsher than necessary and gets in the way of rejecting large folio
> ranges for backend reasons.
> 
> Move the synchronous swapin sequence into a helper and retry with an
> order-0 folio when a large folio cannot be inserted into the swap cache.
> Count the event as an mTHP swapin fallback before dropping the failed
> large allocation.
> 
> Signed-off-by: fujunjie <fujunjie1@xxxxxx>
> ---
>  mm/memory.c | 50 +++++++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 39 insertions(+), 11 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index ea6568571131..84e3b77b8293 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4757,6 +4757,44 @@ static struct folio *alloc_swap_folio(struct vm_fault *vmf)
>  }
>  #endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>  
> +static struct folio *swapin_synchronous_folio(swp_entry_t entry,
> +					      struct vm_fault *vmf)
> +{
> +	struct folio *swapcache, *folio;
> +	bool large;
> +	int order;
> +
> +	folio = alloc_swap_folio(vmf);
> +	if (!folio)
> +		return NULL;
> +
> +	large = folio_test_large(folio);
> +	order = folio_order(folio);
> +
> +	/*
> +	 * folio is charged, so swapin can only fail due to raced swapin and
> +	 * return NULL.
> +	 */
> +	swapcache = swapin_folio(entry, folio);
> +	if (swapcache == folio)
> +		return folio;
> +
> +	if (!swapcache && large)
> +		count_mthp_stat(order, MTHP_STAT_SWPIN_FALLBACK);
> +	folio_put(folio);
> +	if (swapcache || !large)
> +		return swapcache;
> +
> +	folio = __alloc_swap_folio(vmf);
> +	if (!folio)
> +		return NULL;
> +
> +	swapcache = swapin_folio(entry, folio);
> +	if (swapcache != folio)
> +		folio_put(folio);
> +	return swapcache;
> +}
> +
>  /* Sanity check that a folio is fully exclusive */
>  static void check_swap_exclusive(struct folio *folio, swp_entry_t entry,
>  				 unsigned int nr_pages)
> @@ -4860,17 +4898,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
>  		swap_update_readahead(folio, vma, vmf->address);
>  	if (!folio) {
>  		if (data_race(si->flags & SWP_SYNCHRONOUS_IO)) {
> -			folio = alloc_swap_folio(vmf);
> -			if (folio) {
> -				/*
> -				 * folio is charged, so swapin can only fail due
> -				 * to raced swapin and return NULL.
> -				 */
> -				swapcache = swapin_folio(entry, folio);
> -				if (swapcache != folio)
> -					folio_put(folio);
> -				folio = swapcache;
> -			}
> +			folio = swapin_synchronous_folio(entry, vmf);
>  		} else {
>  			folio = swapin_readahead(entry, GFP_HIGHUSER_MOVABLE, vmf);
>  		}

There are some upcoming changes with:

https://lore.kernel.org/r/20260421-swap-table-p4-v3-5-2f23759a76bc@xxxxxxxxxxx


All the of that logic you have in swapin_synchronous_folio() should ideally not
go into memory.c, but into some swap specific code.

But

https://lore.kernel.org/r/20260421-swap-table-p4-v3-0-2f23759a76bc@xxxxxxxxxxx

Already changes a lot of that.

-- 
Cheers,

David