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mm/hmm: add default fault flags to avoid the need to pre-fill pfns arrays

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The HMM mirror API can be use in two fashions.  The first one where the
HMM user coalesce multiple page faults into one request and set flags per
pfns for of those faults.  The second one where the HMM user want to
pre-fault a range with specific flags.  For the latter one it is a waste
to have the user pre-fill the pfn arrays with a default flags value.

This patch adds a default flags value allowing user to set them for a
range without having to pre-fill the pfn array.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jérôme Glisse <[email protected]>
Reviewed-by: Ralph Campbell <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Balbir Singh <[email protected]>
Cc: Dan Carpenter <[email protected]>
Cc: Ira Weiny <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Souptick Joarder <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Jérôme Glisse 2019-05-13 17:20:05 -07:00 • committed by Linus Torvalds
commit 023a019a9b
3 changed files with 60 additions and 0 deletions
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35
Documentation/vm/hmm.rst
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@ -276,6 +276,41 @@ report commands as executed is serialized (there is no point in doing this
concurrently).
Leverage default_flags and pfn_flags_mask
=========================================
The hmm_range struct has 2 fields default_flags and pfn_flags_mask that allows
to set fault or snapshot policy for a whole range instead of having to set them
for each entries in the range.
For instance if the device flags for device entries are:
VALID (1 << 63)
WRITE (1 << 62)
Now let say that device driver wants to fault with at least read a range then
it does set:
range->default_flags = (1 << 63)
range->pfn_flags_mask = 0;
and calls hmm_range_fault() as described above. This will fill fault all page
in the range with at least read permission.
Now let say driver wants to do the same except for one page in the range for
which its want to have write. Now driver set:
range->default_flags = (1 << 63);
range->pfn_flags_mask = (1 << 62);
range->pfns[index_of_write] = (1 << 62);
With this HMM will fault in all page with at least read (ie valid) and for the
address == range->start + (index_of_write << PAGE_SHIFT) it will fault with
write permission ie if the CPU pte does not have write permission set then HMM
will call handle_mm_fault().
Note that HMM will populate the pfns array with write permission for any entry
that have write permission within the CPU pte no matter what are the values set
in default_flags or pfn_flags_mask.
Represent and manage device memory from core kernel point of view
=================================================================