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    SubjectRe: [PATCH v4 2/3] mm: introduce put_user_page*(), placeholder versions
    On Thu 11-10-18 20:53:34, John Hubbard wrote:
    > On 10/11/18 6:23 PM, John Hubbard wrote:
    > > On 10/11/18 6:20 AM, Jason Gunthorpe wrote:
    > >> On Thu, Oct 11, 2018 at 10:49:29AM +0200, Jan Kara wrote:
    > >>
    > >>>> This is a real worry. If someone uses a mistaken put_page() then how
    > >>>> will that bug manifest at runtime? Under what set of circumstances
    > >>>> will the kernel trigger the bug?
    > >>>
    > >>> At runtime such bug will manifest as a page that can never be evicted from
    > >>> memory. We could warn in put_page() if page reference count drops below
    > >>> bare minimum for given user pin count which would be able to catch some
    > >>> issues but it won't be 100% reliable. So at this point I'm more leaning
    > >>> towards making get_user_pages() return a different type than just
    > >>> struct page * to make it much harder for refcount to go wrong...
    > >>
    > >> At least for the infiniband code being used as an example here we take
    > >> the struct page from get_user_pages, then stick it in a sgl, and at
    > >> put_page time we get the page back out of the sgl via sg_page()
    > >>
    > >> So type safety will not help this case... I wonder how many other
    > >> users are similar? I think this is a pretty reasonable flow for DMA
    > >> with user pages.
    > >>
    > >
    > > That is true. The infiniband code, fortunately, never mixes the two page
    > > types into the same pool (or sg list), so it's actually an easier example
    > > than some other subsystems. But, yes, type safety doesn't help there. I can
    > > take a moment to look around at the other areas, to quantify how much a type
    > > safety change might help.
    > >
    > > Back to page flags again, out of desperation:
    > >
    > > How much do we know about the page types that all of these subsystems
    > > use? In other words, can we, for example, use bit 1 of page->lru.next (see [1]
    > > for context) as the "dma-pinned" page flag, while tracking pages within parts
    > > of the kernel that call a mix of alloc_pages, get_user_pages, and other allocators?
    > > In order for that to work, page->index, page->private, and bit 1 of page->mapping
    > > must not be used. I doubt that this is always going to hold, but...does it?
    > >
    >
    > Oops, pardon me, please ignore that nonsense about page->index and page->private
    > and page->mapping, that's actually fine (I was seeing "union", where "struct" was
    > written--too much staring at this code).
    >
    > So actually, I think maybe we can just use bit 1 in page->lru.next to sort out
    > which pages are dma-pinned, in the calling code, just like we're going to do
    > in writeback situations. This should also allow run-time checking that Andrew was
    > hoping for:
    >
    > put_user_page(): assert that the page is dma-pinned
    > put_page(): assert that the page is *not* dma-pinned
    >
    > ...both of which depend on that bit being, essentially, available as sort
    > of a general page flag. And in fact, if it's not, then the whole approach
    > is dead anyway.

    Well, put_page() cannot assert page is not dma-pinned as someone can still
    to get_page(), put_page() on dma-pinned page and that must not barf. But
    put_page() could assert that if the page is pinned, refcount is >=
    pincount. That will detect leaked pin references relatively quickly.

    Honza
    --
    Jan Kara <jack@suse.com>
    SUSE Labs, CR

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