On Mon, Dec 16, 2019 at 02:25:18PM -0800, John Hubbard wrote:
As it says in the updated comment in gup.c: current FOLL_LONGTERM behavior is incompatible with FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on vmas.
However, the corresponding restriction in get_user_pages_remote() was slightly stricter than is actually required: it forbade all FOLL_LONGTERM callers, but we can actually allow FOLL_LONGTERM callers that do not set the "locked" arg.
Update the code and comments to loosen the restriction, allowing FOLL_LONGTERM in some cases.
Also, copy the DAX check ("if a VMA is DAX, don't allow long term pinning") from the VFIO call site, all the way into the internals of get_user_pages_remote() and __gup_longterm_locked(). That is: get_user_pages_remote() calls __gup_longterm_locked(), which in turn calls check_dax_vmas(). This check will then be removed from the VFIO call site in a subsequent patch.
Thanks to Jason Gunthorpe for pointing out a clean way to fix this, and to Dan Williams for helping clarify the DAX refactoring.
Tested-by: Alex Williamson alex.williamson@redhat.com Acked-by: Alex Williamson alex.williamson@redhat.com Reviewed-by: Jason Gunthorpe jgg@mellanox.com Reviewed-by: Ira Weiny ira.weiny@intel.com Suggested-by: Jason Gunthorpe jgg@ziepe.ca Cc: Dan Williams dan.j.williams@intel.com Cc: Jerome Glisse jglisse@redhat.com Signed-off-by: John Hubbard jhubbard@nvidia.com
mm/gup.c | 27 ++++++++++++++++++++++----- 1 file changed, 22 insertions(+), 5 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c index 3ecce297a47f..c0c56888e7cc 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -29,6 +29,13 @@ struct follow_page_context { unsigned int page_mask; }; +static __always_inline long __gup_longterm_locked(struct task_struct *tsk,
struct mm_struct *mm,unsigned long start,unsigned long nr_pages,struct page **pages,struct vm_area_struct **vmas,unsigned int flags);
Any particular reason for the forward declaration? Maybe move get_user_pages_remote() down?
/*
- Return the compound head page with ref appropriately incremented,
- or NULL if that failed.
@@ -1179,13 +1186,23 @@ long get_user_pages_remote(struct task_struct *tsk, struct mm_struct *mm, struct vm_area_struct **vmas, int *locked) { /*
* FIXME: Current FOLL_LONGTERM behavior is incompatible with
* Parts of FOLL_LONGTERM behavior are incompatible with
- FAULT_FLAG_ALLOW_RETRY because of the FS DAX check requirement on
* vmas. As there are no users of this flag in this call we simply* disallow this option for now.
* vmas. However, this only comes up if locked is set, and there are* callers that do request FOLL_LONGTERM, but do not set locked. So,* allow what we can.
- if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
return -EINVAL;
- if (gup_flags & FOLL_LONGTERM) {
if (WARN_ON_ONCE(locked))return -EINVAL;/** This will check the vmas (even if our vmas arg is NULL)* and return -ENOTSUPP if DAX isn't allowed in this case:*/return __gup_longterm_locked(tsk, mm, start, nr_pages, pages,vmas, gup_flags | FOLL_TOUCH |FOLL_REMOTE);- }
return __get_user_pages_locked(tsk, mm, start, nr_pages, pages, vmas, locked, -- 2.24.1