Re: [PATCH v2] mm/gup: fix gup_fast with dynamic page table folding

[Jason Gunthorpe <jgg@ziepe.ca>]

On Fri, Sep 11, 2020 at 10:36:43PM +0200, Vasily Gorbik wrote:
> Currently to make sure that every page table entry is read just once
> gup_fast walks perform READ_ONCE and pass pXd value down to the next
> gup_pXd_range function by value e.g.:
> 
> static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
>                          unsigned int flags, struct page **pages, int *nr)
> ...
>         pudp = pud_offset(&p4d, addr);
> 
> This function passes a reference on that local value copy to pXd_offset,
> and might get the very same pointer in return. This happens when the
> level is folded (on most arches), and that pointer should not be iterated.
> 
> On s390 due to the fact that each task might have different 5,4 or
> 3-level address translation and hence different levels folded the logic
> is more complex and non-iteratable pointer to a local copy leads to
> severe problems.
> 
> Here is an example of what happens with gup_fast on s390, for a task
> with 3-levels paging, crossing a 2 GB pud boundary:
> 
> // addr = 0x1007ffff000, end = 0x10080001000
> static int gup_pud_range(p4d_t p4d, unsigned long addr, unsigned long end,
>                          unsigned int flags, struct page **pages, int *nr)
> {
>         unsigned long next;
>         pud_t *pudp;
> 
>         // pud_offset returns &p4d itself (a pointer to a value on stack)
>         pudp = pud_offset(&p4d, addr);
>         do {
>                 // on second iteratation reading "random" stack value
>                 pud_t pud = READ_ONCE(*pudp);
> 
>                 // next = 0x10080000000, due to PUD_SIZE/MASK != PGDIR_SIZE/MASK on s390
>                 next = pud_addr_end(addr, end);
>                 ...
>         } while (pudp++, addr = next, addr != end); // pudp++ iterating over stack
> 
>         return 1;
> }
> 
> This happens since s390 moved to common gup code with
> commit d1874a0c2805 ("s390/mm: make the pxd_offset functions more robust")
> and commit 1a42010cdc26 ("s390/mm: convert to the generic
> get_user_pages_fast code"). s390 tried to mimic static level folding by
> changing pXd_offset primitives to always calculate top level page table
> offset in pgd_offset and just return the value passed when pXd_offset
> has to act as folded.
> 
> What is crucial for gup_fast and what has been overlooked is
> that PxD_SIZE/MASK and thus pXd_addr_end should also change
> correspondingly. And the latter is not possible with dynamic folding.
> 
> To fix the issue in addition to pXd values pass original
> pXdp pointers down to gup_pXd_range functions. And introduce
> pXd_offset_lockless helpers, which take an additional pXd
> entry value parameter. This has already been discussed in
> https://lkml.kernel.org/r/20190418100218.0a4afd51@mschwideX1
> 
> Cc: <stable@vger.kernel.org> # 5.2+
> Fixes: 1a42010cdc26 ("s390/mm: convert to the generic get_user_pages_fast code")
> Reviewed-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
> Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
> Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
> ---
> v2: added brackets &pgd -> &(pgd)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

Regards,
Jason

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