linux-kernel - Re: [PATCH v2 04/40] mm/rmap: introduce and use hugetlb_try_dup_anon

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Date: Thu, 21 Dec 2023 10:29:09 +0100
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
 Andrew Morton <akpm@...ux-foundation.org>, Hugh Dickins <hughd@...gle.com>,
 Ryan Roberts <ryan.roberts@....com>, Yin Fengwei <fengwei.yin@...el.com>,
 Mike Kravetz <mike.kravetz@...cle.com>, Muchun Song <muchun.song@...ux.dev>,
 Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v2 04/40] mm/rmap: introduce and use
 hugetlb_try_dup_anon_rmap()

On 21.12.23 05:40, Matthew Wilcox wrote:
> On Wed, Dec 20, 2023 at 11:44:28PM +0100, David Hildenbrand wrote:
>> hugetlb rmap handling differs quite a lot from "ordinary" rmap code.
>> For example, hugetlb currently only supports entire mappings, and treats
>> any mapping as mapped using a single "logical PTE". Let's move it out
>> of the way so we can overhaul our "ordinary" rmap.
>> implementation/interface.
>>
>> So let's introduce and use hugetlb_try_dup_anon_rmap() to make all
>> hugetlb handling use dedicated hugetlb_* rmap functions.
>>
>> Add sanity checks that we end up with the right folios in the right
>> functions.
>>
>> Note that is_device_private_page() does not apply to hugetlb.
>>
>> Reviewed-by: Yin Fengwei <fengwei.yin@...el.com>
>> Reviewed-by: Ryan Roberts <ryan.roberts@....com>
>> Signed-off-by: David Hildenbrand <david@...hat.com>
> 
> Reviewed-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> 

Thanks!

>> +static inline bool folio_needs_cow_for_dma(struct vm_area_struct *vma,
>> +					  struct folio *folio)
> 
> I particularly like it that you introduced this.

And a later patch even removes page_needs_cow_for_dma() :)


A note that we have one remaining user of page_maybe_dma_pinned(). 
Instead of converting that code to folios, we should probably just 
remove that pte_is_pinned() handling completely: it's inconsistent (only 
checks PTEs) and cannot handle concurrent GUP-fast. It's a leftover from 
the COW issues we had before PageAnonExclusive. [I've had patch lying 
around to do that for a long time, but never sent it]

> 
>> +static inline int hugetlb_try_dup_anon_rmap(struct folio *folio,
>> +		struct vm_area_struct *vma)
>> +{
>> +	VM_WARN_ON_FOLIO(!folio_test_hugetlb(folio), folio);
>> +	VM_WARN_ON_FOLIO(!folio_test_anon(folio), folio);
>> +
>> +	if (PageAnonExclusive(&folio->page)) {
> 
> I wonder if we need a folio_test_hugetlb_anon_exclusive() to make this
> a little more ergonomic?
> 
>> +		if (unlikely(folio_needs_cow_for_dma(vma, folio)))
>> +			return -EBUSY;
>> +		ClearPageAnonExclusive(&folio->page);
> 
> ... and set/clear variants.
> 

I thought about that as well, and even going a step further and instead 
of having PageAnonExclusive checks outside rmap code, have something 
like the following instead:

hugetlb_test_anon_rmap_exclusive()
folio_test_anon_rmap_exclusive_[pte|pmd]()

I added that to my TODO list, because it results again in a bigger 
patchset (especially also in GUP).

-- 
Cheers,

David / dhildenb