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    SubjectRe: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages
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    On 3/24/21 1:24 PM, Jason Gunthorpe wrote:
    > On Wed, Mar 24, 2021 at 10:56:43AM +0100, Daniel Vetter wrote:
    >> On Tue, Mar 23, 2021 at 06:06:53PM +0100, Thomas Hellström (Intel) wrote:
    >>> On 3/23/21 5:37 PM, Jason Gunthorpe wrote:
    >>>> On Tue, Mar 23, 2021 at 05:34:51PM +0100, Thomas Hellström (Intel) wrote:
    >>>>
    >>>>>>> @@ -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
    >>>>> Yes, that should work.
    >>>>>> - 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 gup_fast will unfortunately mistake a huge iomem page for an
    >>>>> ordinary page and try to access a non-existant struct page for it, unless we
    >>>>> do the devmap trick.
    >>>>>
    >>>>> And the lookup would then be for the rare case where a driver would have
    >>>>> already registered a dev_pagemap for an iomem area which may also be mapped
    >>>>> through TTM (like the patch from Felix a couple of weeks ago). If a driver
    >>>>> can promise not to do that, then we can safely remove the lookup.
    >>>> Isn't the devmap PTE flag arch optional? Does this fall back to not
    >>>> using huge pages on arches that don't support it?
    >>> Good point. No, currently it's only conditioned on transhuge page support.
    >>> Need to condition it on also devmap support.
    >>>
    >>>> Also, I feel like this code to install "pte_special" huge pages does
    >>>> not belong in the drm subsystem..
    >>> I could add helpers in huge_memory.c:
    >>>
    >>> vmf_insert_pfn_pmd_prot_special() and
    >>> vmf_insert_pfn_pud_prot_special()
    >> The somewhat annoying thing is that we'd need an error code so we fall
    >> back to pte fault handling. That's at least my understanding of how
    >> pud/pmd fault handling works. Not sure how awkward that is going to be
    >> with the overall fault handling flow.
    >>
    >> But aside from that I think this makes tons of sense.
    > Why should the driver be so specific?
    >
    > vmf_insert_pfn_range_XXX()
    >
    > And it will figure out the optimal way to build the page tables.
    >
    > Driver should provide the largest physically contiguous range it can

    I figure that would probably work, but since the huge_fault() interface
    is already providing the size of the fault based on how the pagetable is
    currently populated I figure that would have to move a lot of that logic
    into that helper...

    /Thomas


    >
    > Jason

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