Re: [PATCH 1/1] mm/madvise: enhance lazyfreeing with mTHP in madvise_free

[Ryan Roberts]

On 26/02/2024 08:35, Lance Yang wrote:
> Hey Fengwei,
> 
> Thanks for taking time to review!
> 
>> On Mon, Feb 26, 2024 at 10:38 AM Yin Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>>> On Sun, Feb 25, 2024 at 8:32 PM Lance Yang <ioworker0@xxxxxxxxx> wrote:
> [...]
>>> --- a/mm/madvise.c
>>> +++ b/mm/madvise.c
>>> @@ -676,11 +676,43 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>>>                */
>>>               if (folio_test_large(folio)) {
>>>                       int err;
>>> +                     unsigned long next_addr, align;
>>>
>>> -                     if (folio_estimated_sharers(folio) != 1)
>>> -                             break;
>>> -                     if (!folio_trylock(folio))
>>> -                             break;
>>> +                     if (folio_estimated_sharers(folio) != 1 ||
>>> +                         !folio_trylock(folio))
>>> +                             goto skip_large_folio;
>>> +
>>> +                     align = folio_nr_pages(folio) * PAGE_SIZE;
>>> +                     next_addr = ALIGN_DOWN(addr + align, align);
>> There is a possible corner case:
>> If there is a cow folio associated with this folio and the cow folio
>> has smaller size than this folio for whatever reason, this change can't
>> handle it correctly.
> 
> Thanks for pointing that out; it's very helpful to me!
> I made some changes. Could you please check if this corner case is now resolved?
> 
> As a diff against this patch.
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index bcbf56595a2e..c7aacc9f9536 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -686,10 +686,12 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
>  			next_addr = ALIGN_DOWN(addr + align, align);
>  
>  			/*
> -			 * If we mark only the subpages as lazyfree,
> -			 * split the large folio.
> +			 * If we mark only the subpages as lazyfree, or
> +			 * if there is a cow folio associated with this folio,
> +			 * then split the large folio.
>  			 */
> -			if (next_addr > end || next_addr - addr != align)
> +			if (next_addr > end || next_addr - addr != align ||
> +			    folio_total_mapcount(folio) != folio_nr_pages(folio))

I still don't think this is correct. I think you were previously assuming that
if you see a page from a large folio then the whole large folio should be
contiguously mapped? This new check doesn't validate that assumption reliably;
you need to iterate through every pte to generate a batch, like David does in
folio_pte_batch() for this to be safe.

An example of when this check is insufficient; let's say you have a 4 page anon
folio mapped contiguously in a process (total_mapcount=4). The process is forked
(total_mapcount=8). Then each process munmaps the second 2 pages
(total_mapcount=4). In place of the munmapped 2 pages, 2 new pages are mapped.
Then call madvise. It's probably even easier to trigger for file-backed memory
(I think this code path is used for both file and anon?)

Thanks,
Ryan




>  				goto split_large_folio;
>  
>  			/*
> ---
> 
> Thanks again for your time!
> 
> Best,
> Lance