Re: [PATCH v8 19/20] fs/dax: Properly refcount fs dax pages

David Hildenbrand
2025-02-20T11:52:07Z

>>> -static inline unsigned long dax_folio_share_put(struct folio *folio)
>>> +static inline unsigned long dax_folio_put(struct folio *folio)
>>> {
>>> - return --folio->page.share;
>>> + unsigned long ref;
>>> + int order, i;
>>> +
>>> + if (!dax_folio_is_shared(folio))
>>> + ref = 0;
>>> + else
>>> + ref = --folio->share;
>>> +
>>
>> out of interest, what synchronizes access to folio->share?
>
> Actually that's an excellent question as I hadn't looked too closely at this
> given I wasn't changing the overall flow with regards to synchronization, merely
> representation of the "shared" state. So I don't have a good answer for you off
> the top of my head - Dan maybe you can shed some light here?

Not that I understand what that dax-shared thing is or does, but the
non-atomic update on a folio_put path looked "surprising".

>>> diff --git a/include/linux/dax.h b/include/linux/dax.h
>>> index 2333c30..dcc9fcd 100644
>>> --- a/include/linux/dax.h
>>> +++ b/include/linux/dax.h
>>> @@ -209,7 +209,7 @@ int dax_truncate_page(struct inode *inode, loff_t pos, bool *did_zero,
>>
>> [...]
>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index d189826..1a0d6a8 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -2225,7 +2225,7 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>>> tlb->fullmm);
>>> arch_check_zapped_pmd(vma, orig_pmd);
>>> tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
>>> - if (vma_is_special_huge(vma)) {
>>> + if (!vma_is_dax(vma) && vma_is_special_huge(vma)) {
>>
>> I wonder if we actually want to remove the vma_is_dax() check from
>> vma_is_special_huge(), and instead add it to the remaining callers of
>> vma_is_special_huge() that still need it -- if any need it.
>>
>> Did we sanity-check which callers of vma_is_special_huge() still need it? Is
>> there still reason to have that DAX check in vma_is_special_huge()?
>
> If by "we" you mean "me" then yes :) There are still a few callers of it, mainly
> for page splitting.

Heh, "you or any of the reviewers" :)

So IIUC, the existing users still need the DAX check I assume (until
that part is cleaned up, below).

>
>> But vma_is_special_huge() is rather confusing from me ... the whole
>> vma_is_special_huge() thing should probably be removed. That's a cleanup for
>> another day, though.
>
> But after double checking I have come to the same conclusion as you - it should
> be removed. I will add that to my ever growing clean-up series that can go on
> top of this one.

Nice!

--
Cheers,

David / dhildenb