hi, Ryan Roberts
在 2024/3/12 20:34, Ryan Roberts 写道:
> On 04/03/2024 08:13, Barry Song wrote:
>> From: Chuanhua Han <hanchuanhua@xxxxxxxx>
>>
>> should_try_to_free_swap() works with an assumption that swap-in is always done
>> at normal page granularity, aka, folio_nr_pages = 1. To support large folio
>> swap-in, this patch removes the assumption.
>>
>> Signed-off-by: Chuanhua Han <hanchuanhua@xxxxxxxx>
>> Co-developed-by: Barry Song <v-songbaohua@xxxxxxxx>
>> Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
>> Acked-by: Chris Li <chrisl@xxxxxxxxxx>
>> ---
>> mm/memory.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index abd4f33d62c9..e0d34d705e07 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -3837,7 +3837,7 @@ static inline bool should_try_to_free_swap(struct folio *folio,
>> * reference only in case it's likely that we'll be the exlusive user.
>> */
>> return (fault_flags & FAULT_FLAG_WRITE) && !folio_test_ksm(folio) &&
>> - folio_ref_count(folio) == 2;
>> + folio_ref_count(folio) == (1 + folio_nr_pages(folio));
> I don't think this is correct; one reference has just been added to the folio in
> do_swap_page(), either by getting from swapcache (swap_cache_get_folio()) or by
> allocating. If it came from the swapcache, it could be a large folio, because we
> swapped out a large folio and never removed it from swapcache. But in that case,
> others may have partially mapped it, so the refcount could legitimately equal
> the number of pages while still not being exclusively mapped.
>
> I'm guessing this logic is trying to estimate when we are likely exclusive so
> that we remove from swapcache (release ref) and can then reuse rather than CoW
> the folio? The main CoW path currently CoWs page-by-page even for large folios,
> and with Barry's recent patch, even the last page gets copied. So not sure what
> this change is really trying to achieve?
>
First, if it is a large folio in the swap cache, then its refcont is at
least folio_nr_pages(folio) :
For example, in add_to_swap_cache path:
int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
gfp_t gfp, void **shadowp)
{
struct address_space *address_space = swap_address_space(entry);
pgoff_t idx = swp_offset(entry);
XA_STATE_ORDER(xas, &address_space->i_pages, idx,
folio_order(folio));
unsigned long i, nr = folio_nr_pages(folio); <---
void *old;
...
folio_ref_add(folio, nr); <---
folio_set_swapcache(folio);
...
}
*
Then in the do_swap_page path:
* if (should_try_to_free_swap(folio, vma, vmf->flags))
folio_free_swap(folio);
*
* It also indicates that only folio in the swap cache will call
folio_free_swap
* to delete it from the swap cache, So I feel like this patch is
necessary!? 😁
>> }
>>
>> static vm_fault_t pte_marker_clear(struct vm_fault *vmf)
Thanks,
Chuanhua
