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Message-ID: <20250613081613.0bef3d39.alex.williamson@redhat.com>
Date: Fri, 13 Jun 2025 08:16:13 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: David Hildenbrand <david@...hat.com>
Cc: lizhe.67@...edance.com, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, peterx@...hat.com
Subject: Re: [RFC v2] vfio/type1: optimize vfio_unpin_pages_remote() for
large folio
On Fri, 13 Jun 2025 15:37:40 +0200
David Hildenbrand <david@...hat.com> wrote:
> On 13.06.25 08:29, lizhe.67@...edance.com wrote:
> > On Thu, 12 Jun 2025 16:32:39 -0600, alex.williamson@...hat.com wrote:
> >
> >>> drivers/vfio/vfio_iommu_type1.c | 53 +++++++++++++++++++++++++--------
> >>> 1 file changed, 41 insertions(+), 12 deletions(-)
> >>>
> >>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> >>> index 28ee4b8d39ae..2f6c0074d7b3 100644
> >>> --- a/drivers/vfio/vfio_iommu_type1.c
> >>> +++ b/drivers/vfio/vfio_iommu_type1.c
> >>> @@ -469,17 +469,28 @@ static bool is_invalid_reserved_pfn(unsigned long pfn)
> >>> return true;
> >>> }
> >>>
> >>> -static int put_pfn(unsigned long pfn, int prot)
> >>> +static inline void _put_pfns(struct page *page, int npages, int prot)
> >>> {
> >>> - if (!is_invalid_reserved_pfn(pfn)) {
> >>> - struct page *page = pfn_to_page(pfn);
> >>> + unpin_user_page_range_dirty_lock(page, npages, prot & IOMMU_WRITE);
> >>> +}
> >>>
> >>> - unpin_user_pages_dirty_lock(&page, 1, prot & IOMMU_WRITE);
> >>> - return 1;
> >>> +/*
> >>> + * The caller must ensure that these npages PFNs belong to the same folio.
> >>> + */
> >>> +static inline int put_pfns(unsigned long pfn, int npages, int prot)
> >>> +{
> >>> + if (!is_invalid_reserved_pfn(pfn)) {
> >>> + _put_pfns(pfn_to_page(pfn), npages, prot);
> >>> + return npages;
> >>> }
> >>> return 0;
> >>> }
> >>>
> >>> +static inline int put_pfn(unsigned long pfn, int prot)
> >>> +{
> >>> + return put_pfns(pfn, 1, prot);
> >>> +}
> >>> +
> >>> #define VFIO_BATCH_MAX_CAPACITY (PAGE_SIZE / sizeof(struct page *))
> >>>
> >>> static void __vfio_batch_init(struct vfio_batch *batch, bool single)
> >>> @@ -805,15 +816,33 @@ static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
> >>> unsigned long pfn, unsigned long npage,
> >>> bool do_accounting)
> >>> {
> >>> - long unlocked = 0, locked = 0;
> >>> - long i;
> >>> + long unlocked = 0, locked = vpfn_pages(dma, iova, npage);
> >>>
> >>> - for (i = 0; i < npage; i++, iova += PAGE_SIZE) {
> >>> - if (put_pfn(pfn++, dma->prot)) {
> >>> - unlocked++;
> >>> - if (vfio_find_vpfn(dma, iova))
> >>> - locked++;
> >>> + while (npage) {
> >>> + struct folio *folio;
> >>> + struct page *page;
> >>> + long step = 1;
> >>> +
> >>> + if (is_invalid_reserved_pfn(pfn))
> >>> + goto next;
> >>> +
> >>> + page = pfn_to_page(pfn);
> >>> + folio = page_folio(page);
> >>> +
> >>> + if (!folio_test_large(folio)) {
> >>> + _put_pfns(page, 1, dma->prot);
> >>> + } else {
> >>> + step = min_t(long, npage,
> >>> + folio_nr_pages(folio) -
> >>> + folio_page_idx(folio, page));
> >>> + _put_pfns(page, step, dma->prot);
> >>> }
> >>> +
> >>> + unlocked += step;
> >>> +next:
> >>
> >> Usage of @step is inconsistent, goto isn't really necessary either, how
> >> about:
> >>
> >> while (npage) {
> >> unsigned long step = 1;
> >>
> >> if (!is_invalid_reserved_pfn(pfn)) {
> >> struct page *page = pfn_to_page(pfn);
> >> struct folio *folio = page_folio(page);
> >> long nr_pages = folio_nr_pages(folio);
> >>
> >> if (nr_pages > 1)
> >> step = min_t(long, npage,
> >> nr_pages -
> >> folio_page_idx(folio, page));
> >>
> >> _put_pfns(page, step, dma->prot);
> >> unlocked += step;
> >> }
> >>
> >
> > That's great. This implementation is much better.
> >
> > I'm a bit uncertain about the best type to use for the 'step'
> > variable here. I've been trying to keep things consistent with the
> > put_pfn() function, so I set the type of the second parameter in
> > _put_pfns() to 'int'(we pass 'step' as the second argument to
> > _put_pfns()).
> >
> > Using unsigned long for 'step' should definitely work here, as the
> > number of pages in a large folio currently falls within the range
> > that can be represented by an int. However, there is still a
> > potential risk of truncation that we need to be mindful of.
> >
> >>> + pfn += step;
> >>> + iova += PAGE_SIZE * step;
> >>> + npage -= step;
> >>> }
> >>>
> >>> if (do_accounting)
> >>
> >> AIUI, the idea is that we know we have npage contiguous pfns and we
> >> currently test invalid/reserved, call pfn_to_page(), call
> >> unpin_user_pages_dirty_lock(), and test vpfn for each individually.
> >>
> >> This instead wants to batch the vpfn accounted pfns using the range
> >> helper added for the mapping patch,
> >
> > Yes. We use vpfn_pages() just to track the locked pages.
> >
> >> infer that continuous pfns have the
> >> same invalid/reserved state, the pages are sequential, and that we can
> >> use the end of the folio to mark any inflections in those assumptions
> >> otherwise. Do I have that correct?
> >
> > Yes. I think we're definitely on the same page here.
> >
> >> I think this could be split into two patches, one simply batching the
> >> vpfn accounting and the next introducing the folio dependency. The
> >> contributions of each to the overall performance improvement would be
> >> interesting.
> >
> > I've made an initial attempt, and here are the two patches after
> > splitting them up.
> >
> > 1. batch-vpfn-accounting-patch:
> >
> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index 28ee4b8d39ae..c8ddcee5aa68 100644
> > --- a/drivers/vfio/vfio_iommu_type1.c
> > +++ b/drivers/vfio/vfio_iommu_type1.c
> > @@ -805,16 +805,12 @@ static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
> > unsigned long pfn, unsigned long npage,
> > bool do_accounting)
> > {
> > - long unlocked = 0, locked = 0;
> > + long unlocked = 0, locked = vpfn_pages(dma, iova, npage);
> > long i;
> >
> > - for (i = 0; i < npage; i++, iova += PAGE_SIZE) {
> > - if (put_pfn(pfn++, dma->prot)) {
> > + for (i = 0; i < npage; i++, iova += PAGE_SIZE)
> > + if (put_pfn(pfn++, dma->prot))
> > unlocked++;
> > - if (vfio_find_vpfn(dma, iova))
> > - locked++;
> > - }
> > - }
> >
> > if (do_accounting)
> > vfio_lock_acct(dma, locked - unlocked, true);
> > -----------------
> >
> > 2. large-folio-optimization-patch:
> >
> > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > index c8ddcee5aa68..48c2ba4ba4eb 100644
> > --- a/drivers/vfio/vfio_iommu_type1.c
> > +++ b/drivers/vfio/vfio_iommu_type1.c
> > @@ -469,17 +469,28 @@ static bool is_invalid_reserved_pfn(unsigned long pfn)
> > return true;
> > }
> >
> > -static int put_pfn(unsigned long pfn, int prot)
> > +static inline void _put_pfns(struct page *page, int npages, int prot)
> > {
> > - if (!is_invalid_reserved_pfn(pfn)) {
> > - struct page *page = pfn_to_page(pfn);
> > + unpin_user_page_range_dirty_lock(page, npages, prot & IOMMU_WRITE);
> > +}
> >
> > - unpin_user_pages_dirty_lock(&page, 1, prot & IOMMU_WRITE);
> > - return 1;
> > +/*
> > + * The caller must ensure that these npages PFNs belong to the same folio.
> > + */
> > +static inline int put_pfns(unsigned long pfn, int npages, int prot)
> > +{
> > + if (!is_invalid_reserved_pfn(pfn)) {
> > + _put_pfns(pfn_to_page(pfn), npages, prot);
> > + return npages;
> > }
> > return 0;
> > }
> >
> > +static inline int put_pfn(unsigned long pfn, int prot)
> > +{
> > + return put_pfns(pfn, 1, prot);
> > +}
> > +
> > #define VFIO_BATCH_MAX_CAPACITY (PAGE_SIZE / sizeof(struct page *))
> >
> > static void __vfio_batch_init(struct vfio_batch *batch, bool single)
> > @@ -806,11 +817,28 @@ static long vfio_unpin_pages_remote(struct vfio_dma *dma, dma_addr_t iova,
> > bool do_accounting)
> > {
> > long unlocked = 0, locked = vpfn_pages(dma, iova, npage);
> > - long i;
> >
> > - for (i = 0; i < npage; i++, iova += PAGE_SIZE)
> > - if (put_pfn(pfn++, dma->prot))
> > - unlocked++;
> > + while (npage) {
> > + long step = 1;
> > +
> > + if (!is_invalid_reserved_pfn(pfn)) {
> > + struct page *page = pfn_to_page(pfn);
> > + struct folio *folio = page_folio(page);
> > + long nr_pages = folio_nr_pages(folio);
> > +
> > + if (nr_pages > 1)
> > + step = min_t(long, npage,
> > + nr_pages -
> > + folio_page_idx(folio, page));
> > +
> > + _put_pfns(page, step, dma->prot);
>
> I'm confused, why do we batch pages by looking at the folio, to then
> pass the pages into unpin_user_page_range_dirty_lock?
>
> Why does the folio relationship matter at all here?
>
> Aren't we making the same mistake that we are jumping over pages we
> shouldn't be jumping over, because we assume they belong to that folio?
That's my concern as well. On the mapping side we had an array of
pages from gup and we tested each page in the gup array relative to the
folio pages. I think that's because the gup array could have
non-sequential pages, but aiui the folio should have sequential
pages(?). Here I think we're trying to assume that sequential pfns
results in sequential pages and folios should have sequential pages, so
the folio just gives us a point to look for changes in invalid/reserved.
Is that valid? Thanks,
Alex
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