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Message-ID: <YFnST5VLcEgv9q+s@phenom.ffwll.local>
Date: Tue, 23 Mar 2021 12:34:39 +0100
From: Daniel Vetter <daniel@...ll.ch>
To: Thomas Hellström (Intel)
<thomas_os@...pmail.org>
Cc: dri-devel@...ts.freedesktop.org,
Christian Koenig <christian.koenig@....com>,
David Airlie <airlied@...ux.ie>,
Daniel Vetter <daniel@...ll.ch>,
Andrew Morton <akpm@...ux-foundation.org>,
Jason Gunthorpe <jgg@...dia.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages
On Sun, Mar 21, 2021 at 07:45:28PM +0100, Thomas Hellström (Intel) wrote:
> TTM sets up huge page-table-entries both to system- and device memory,
> and we don't want gup to assume there are always valid backing struct
> pages for these. For PTEs this is handled by setting the pte_special bit,
> but for the huge PUDs and PMDs, we have neither pmd_special nor
> pud_special. Normally, huge TTM entries are identified by looking at
> vma_is_special_huge(), but fast gup can't do that, so as an alternative
> define _devmap entries for which there are no backing dev_pagemap as
> special, update documentation and make huge TTM entries _devmap, after
> verifying that there is no backing dev_pagemap.
>
> One other alternative would be to block TTM huge page-table-entries
> completely, and while currently only vmwgfx use them, they would be
> beneficial to other graphis drivers moving forward as well.
>
> Cc: Christian Koenig <christian.koenig@....com>
> Cc: David Airlie <airlied@...ux.ie>
> Cc: Daniel Vetter <daniel@...ll.ch>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Jason Gunthorpe <jgg@...dia.com>
> Cc: linux-mm@...ck.org
> Cc: dri-devel@...ts.freedesktop.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Thomas Hellström (Intel) <thomas_os@...pmail.org>
> ---
> drivers/gpu/drm/ttm/ttm_bo_vm.c | 17 ++++++++++++++++-
> mm/gup.c | 21 +++++++++++----------
> mm/memremap.c | 5 +++++
> 3 files changed, 32 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> index 6dc96cf66744..1c34983480e5 100644
> --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
> +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
> @@ -195,6 +195,7 @@ static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
> pfn_t pfnt;
> struct ttm_tt *ttm = bo->ttm;
> bool write = vmf->flags & FAULT_FLAG_WRITE;
> + struct dev_pagemap *pagemap;
>
> /* Fault should not cross bo boundary. */
> page_offset &= ~(fault_page_size - 1);
> @@ -210,6 +211,20 @@ static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
> if ((pfn & (fault_page_size - 1)) != 0)
> goto out_fallback;
>
> + /*
> + * Huge entries must be special, that is marking them as devmap
> + * with no backing device map range. If there is a backing
> + * range, Don't insert a huge entry.
> + * If this check turns out to be too much of a performance hit,
> + * we can instead have drivers indicate whether they may have
> + * backing device map ranges and if not, skip this lookup.
> + */
I think we can do this statically:
- if it's system memory we know there's no devmap for it, and we do the
trick to block gup_fast
- if it's iomem, we know gup_fast wont work anyway if don't set PFN_DEV,
so might as well not do that
I think that would cover all cases without this check here?
-Daniel
> + pagemap = get_dev_pagemap(pfn, NULL);
> + if (pagemap) {
> + put_dev_pagemap(pagemap);
> + goto out_fallback;
> + }
> +
> /* Check that memory is contiguous. */
> if (!bo->mem.bus.is_iomem) {
> for (i = 1; i < fault_page_size; ++i) {
> @@ -223,7 +238,7 @@ static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
> }
> }
>
> - pfnt = __pfn_to_pfn_t(pfn, PFN_DEV);
> + pfnt = __pfn_to_pfn_t(pfn, PFN_DEV | PFN_MAP);
> if (fault_page_size == (HPAGE_PMD_SIZE >> PAGE_SHIFT))
> ret = vmf_insert_pfn_pmd_prot(vmf, pfnt, pgprot, write);
> #ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
> diff --git a/mm/gup.c b/mm/gup.c
> index e40579624f10..1b6a127f0bdd 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1993,6 +1993,17 @@ static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
> }
>
> #ifdef CONFIG_ARCH_HAS_PTE_SPECIAL
> +/*
> + * If we can't determine whether or not a pte is special, then fail immediately
> + * for ptes. Note, we can still pin HugeTLB as it is guaranteed not to be
> + * special. For THP, special huge entries are indicated by xxx_devmap()
> + * returning true, but a corresponding call to get_dev_pagemap() will
> + * return NULL.
> + *
> + * For a futex to be placed on a THP tail page, get_futex_key requires a
> + * get_user_pages_fast_only implementation that can pin pages. Thus it's still
> + * useful to have gup_huge_pmd even if we can't operate on ptes.
> + */
> static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> unsigned int flags, struct page **pages, int *nr)
> {
> @@ -2069,16 +2080,6 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> return ret;
> }
> #else
> -
> -/*
> - * If we can't determine whether or not a pte is special, then fail immediately
> - * for ptes. Note, we can still pin HugeTLB and THP as these are guaranteed not
> - * to be special.
> - *
> - * For a futex to be placed on a THP tail page, get_futex_key requires a
> - * get_user_pages_fast_only implementation that can pin pages. Thus it's still
> - * useful to have gup_huge_pmd even if we can't operate on ptes.
> - */
> static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> unsigned int flags, struct page **pages, int *nr)
> {
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 7aa7d6e80ee5..757551cd2a4d 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -471,6 +471,11 @@ void vmem_altmap_free(struct vmem_altmap *altmap, unsigned long nr_pfns)
> *
> * If @pgmap is non-NULL and covers @pfn it will be returned as-is. If @pgmap
> * is non-NULL but does not cover @pfn the reference to it will be released.
> + *
> + * Return: A referenced pointer to a struct dev_pagemap containing @pfn,
> + * or NULL if there was no such pagemap registered. For interpretion
> + * of NULL returns for pfns extracted from valid huge page table entries,
> + * please see gup_pte_range().
> */
> struct dev_pagemap *get_dev_pagemap(unsigned long pfn,
> struct dev_pagemap *pgmap)
> --
> 2.30.2
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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