linux-kernel - Re: [PATCH] gup: optimize longterm pin_user

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Message-ID: <20250530150254.52362-1-lizhe.67@bytedance.com>
Date: Fri, 30 May 2025 23:02:54 +0800
From: lizhe.67@...edance.com
To: david@...hat.com
Cc: akpm@...ux-foundation.org,
	jgg@...pe.ca,
	jhubbard@...dia.com,
	linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	lizhe.67@...edance.com,
	muchun.song@...ux.dev,
	peterx@...hat.com
Subject: Re: [PATCH] gup: optimize longterm pin_user_pages() for large folio

On Fri, 30 May 2025 15:08:06 +0200, david@...hat.com wrote:

> >>> diff --git a/mm/gup.c b/mm/gup.c
> >>> index 84461d384ae2..8c11418036e2 100644
> >>> --- a/mm/gup.c
> >>> +++ b/mm/gup.c
> >>> @@ -2317,6 +2317,25 @@ static void pofs_unpin(struct pages_or_folios *pofs)
> >>>    		unpin_user_pages(pofs->pages, pofs->nr_entries);
> >>>    }
> >>>    
> >>> +static struct folio *pofs_next_folio(struct folio *folio,
> >>> +				struct pages_or_folios *pofs, long *index_ptr)
> >>> +{
> >>> +	long i = *index_ptr + 1;
> >>> +	unsigned long nr_pages = folio_nr_pages(folio);
> >>> +
> >>> +	if (!pofs->has_folios)
> >>> +		while ((i < pofs->nr_entries) &&
> >>> +			/* Is this page part of this folio? */
> >>> +			(folio_page_idx(folio, pofs->pages[i]) < nr_pages))
> >>
> >> passing in a page that does not belong to the folio looks shaky and not
> >> future proof.
> >>
> >> folio_page() == folio
> >>
> >> is cleaner
> > 
> > Yes, this approach is cleaner. However, when obtaining a folio
> > corresponding to a page through the page_folio() interface,
> 
> Right, I meant page_folio().
> 
> > READ_ONCE() is used internally to read from memory, which results
> > in the performance of pin_user_pages() being worse than before.
> 
> See contig_pages in [1] how it can be done using folio_page().
> 
> [1] 
> https://lore.kernel.org/all/20250529064947.38433-1-lizhe.67@bytedance.com/T/#u

Thank you for your suggestion. It is indeed a good idea. I
initially thought along the same lines. However, I found that
the conditions for optimization here are slightly different
from those in contig_pages(). Here, it is only necessary to
ensure that the page is within the folio, rather than
requiring contiguity.

I have made some preliminary attempts: using the method of
contig_pages() still gets an optimization effect of
approximately 73%. On the other hand, if we use the following
code to determine whether page_to_pfn(pofs->pages[i]) belongs
to the range
[folio_pfn(folio), folio_pfn(folio) + folio_nr_pages(folio)),
the optimization effect is about 70%. I sincerely hope to
hear your thoughts on which solution you might favor.

+static struct folio *pofs_next_folio(struct folio *folio,
+		struct pages_or_folios *pofs, long *index_ptr)
+{
+	long i = *index_ptr + 1;
+
+	if (!pofs->has_folios) {
+		unsigned long start_pfn = folio_pfn(folio);
+		unsigned long end_pfn = start_pfn + folio_nr_pages(folio);
+
+		for (; i < pofs->nr_entries; i++) {
+			unsigned long pfn = page_to_pfn(pofs->pages[i]);
+
+			/* Is this page part of this folio? */
+			if ((pfn < start_pfn) || (pfn >= end_pfn))
+				break;
+		}
+	}
+
+	if (unlikely(i == pofs->nr_entries))
+		return NULL;
+	*index_ptr = i;
+
+	return pofs_get_folio(pofs, i);
+}

Thanks,
Zhe