Messages in this thread | | | Subject | Re: [PATCH 09/16] mm/hmm: add output flag for compound page mapping | From | Ralph Campbell <> | Date | Mon, 22 Jun 2020 16:26:41 -0700 |
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On 6/22/20 4:18 PM, Jason Gunthorpe wrote: > On Mon, Jun 22, 2020 at 11:10:05AM -0700, Ralph Campbell wrote: >> >> On 6/22/20 10:25 AM, Jason Gunthorpe wrote: >>> On Fri, Jun 19, 2020 at 02:56:42PM -0700, Ralph Campbell wrote: >>>> hmm_range_fault() returns an array of page frame numbers and flags for >>>> how the pages are mapped in the requested process' page tables. The PFN >>>> can be used to get the struct page with hmm_pfn_to_page() and the page size >>>> order can be determined with compound_order(page) but if the page is larger >>>> than order 0 (PAGE_SIZE), there is no indication that the page is mapped >>>> using a larger page size. To be fully general, hmm_range_fault() would need >>>> to return the mapping size to handle cases like a 1GB compound page being >>>> mapped with 2MB PMD entries. However, the most common case is the mapping >>>> size is the same as the underlying compound page size. >>>> Add a new output flag to indicate this so that callers know it is safe to >>>> use a large device page table mapping if one is available. >>> >>> But what size should the caller use? >>> >>> You already explained that the caller cannot use compound_ordet() to >>> get the size, so what should it be? >>> >>> Probably this needs to be two flags, PUD and PMD, and the caller should >>> use the PUD and PMD sizes to figure out how big it is? >>> >>> Jason >>> >> >> I guess I didn't explain it as clearly as I thought. :-) >> >> The page size *can* be determined with compound_order(page) but without the >> flag, the caller doesn't know how much of that page is being mapped by the >> CPU. The flag says the CPU is mapping the whole compound page (based on compound_order) >> and that the caller can use device mappings up to the size of compound_order(page). > > No, I got it, I just don't like the assumption that just because a PMD > or PUD points to a page that the only possible value for > compound_page() is PMD or PUD respectively. Partial mapping should be > possible in both cases, if not today, then maybe down the road with > some of the large page work that has been floating about > > It seems much safer to just directly encode the PUD/PMD size in the > flags > > Jason
That is fine with me. I'll make that change for v2. I was just trying to minimize the number of flags being added.
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