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

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Message-ID: <20250530100453.36028-1-lizhe.67@bytedance.com>
Date: Fri, 30 May 2025 18:04:53 +0800
From: lizhe.67@...edance.com
To: dev.jain@....com
Cc: akpm@...ux-foundation.org,
	david@...hat.com,
	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,
	ryan.roberts@....com
Subject: Re: [PATCH] gup: optimize longterm pin_user_pages() for large folio

On Fri, 30 May 2025 15:23:49 +0530, dev.jain@....com wrote:

> On 30/05/25 2:53 pm, lizhe.67@...edance.com wrote:
> > From: Li Zhe<lizhe.67@...edance.com>
> >
> > In the current implementation of the longterm pin_user_pages() function,
> > we invoke the collect_longterm_unpinnable_folios() function. This function
> > iterates through the list to check whether each folio belongs to the
> > "longterm_unpinnabled" category. The folios in this list essentially
> > correspond to a contiguous region of user-space addresses, with each folio
> > representing a physical address in increments of PAGESIZE. If this
> > user-space address range is mapped with large folio, we can optimize the
> > performance of function pin_user_pages() by reducing the number of if-else
> > branches and the frequency of memory accesses using READ_ONCE. This patch
> > leverages this approach to achieve performance improvements.
> >
> > The performance test results obtained through the gup_test tool from the
> > kernel source tree are as follows. We achieve an improvement of over 75%
> > for large folio with pagesize=2M. For normal page, we have only observed
> > a very slight degradation in performance.
> 
> 
> Thanks for the patch! I have no idea on GUP but in my limited understanding
> the patch looks fine, let's wait for more comments.
> 
> [----]
> 
> >   }
> >   
> > +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))
> > +			i++;
> > +
> > +	if (unlikely(i == pofs->nr_entries))
> > +		return NULL;
> > +	*index_ptr = i;
> > +
> > +	return pofs_get_folio(pofs, i);
> > +}
> > +
> >   /*
> >    * Returns the number of collected folios. Return value is always >= 0.
> >    */
> > @@ -2324,16 +2343,12 @@ static void collect_longterm_unpinnable_folios(
> >   		struct list_head *movable_folio_list,
> >   		struct pages_or_folios *pofs)
> >   {
> > -	struct folio *prev_folio = NULL;
> >   	bool drain_allow = true;
> > -	unsigned long i;
> > -
> > -	for (i = 0; i < pofs->nr_entries; i++) {
> > -		struct folio *folio = pofs_get_folio(pofs, i);
> > +	long i = 0;
> 
> 
> Why not unsigned long?

This is because I want to match the type of pages_or_folios->nr_entries.

Thanks,
Zhe