On 21.02.25 13:05, Lorenzo Stoakes wrote:
Currently there is no means by which users can determine whether a given
page in memory is in fact a guard region, that is having had the
MADV_GUARD_INSTALL madvise() flag applied to it.
This is intentional, as to provide this information in VMA metadata would
contradict the intent of the feature (providing a means to change fault
behaviour at a page table level rather than a VMA level), and would require
VMA metadata operations to scan page tables, which is unacceptable.
In many cases, users have no need to reflect and determine what regions
have been designated guard regions, as it is the user who has established
them in the first place.
But in some instances, such as monitoring software, or software that relies
upon being able to ascertain the nature of mappings within a remote process
for instance, it becomes useful to be able to determine which pages have
the guard region marker applied.
This patch makes use of an unused pagemap bit (58) to provide this
information.
This patch updates the documentation at the same time as making the change
such that the implementation of the feature and the documentation of it are
tied together.
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
---
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
Something that might be interesting is also extending the PAGEMAP_SCAN
ioctl.
See do_pagemap_scan().
The benefit here might be that one could effectively search/filter for
guard regions without copying 64bit per base-page to user space.
But the idea would be to indicate something like PAGE_IS_GUARD_REGION as
a category when we hit a guard region entry in pagemap_page_category().
(the code is a bit complicated, and I am not sure why we indicate
PAGE_IS_SWAPPED for non-swap entries, likely wrong ...)
--
Cheers,
David / dhildenb
