On 24.02.25 11:18, Lorenzo Stoakes wrote:
On Mon, Feb 24, 2025 at 10:27:28AM +0100, David Hildenbrand wrote:
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>
Thanks! :)
Something that might be interesting is also extending the PAGEMAP_SCAN
ioctl.
Yeah, funny you should mention that, I did see that, but on reading the man
page it struck me that it requires the region to be uffd afaict? All the
tests seem to establish uffd, and the man page implies it:
To start tracking the written state (flag) of a page or range of
memory, the UFFD_FEATURE_WP_ASYNC must be enabled by UFFDIO_API
ioctl(2) on userfaultfd and memory range must be registered with
UFFDIO_REGISTER ioctl(2) in UFFDIO_REGISTER_MODE_WP mode.
It would be a bit of a weird edge case to add support there. I was excited
when I first saw this ioctl, then disappointed afterwards... but maybe I
got it wrong?
I never managed to review that fully, but I thing that
UFFD_FEATURE_WP_ASYNC thingy is only required for PM_SCAN_CHECK_WPASYNC
and PM_SCAN_WP_MATCHING.
See pagemap_scan_test_walk().
I do recall that it works on any VMA.
Ah yes, tools/testing/selftests/mm/vm_util.c ends up using it for
pagemap_is_swapped() and friends via page_entry_is() to sanity check
that what pagemap gives us is consistent with what pagemap_scan gives us.
So it should work independent of the uffd magic.
I might be wrong, though ...
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 ...)
Yeah, I could go on here about how much I hate how uffd does a 'parallel
implementation' of a ton of stuff and then chucks in if (uffd) { go do
something weird + wonderful } but I'll resist the urge :P :))
Do you think, if it were uffd-specific, this would be useful?
If it really is completely uffd-specific for now, I agree that we should
rather leave it alone.
At any rate, I'm not sure it's _hugely_ beneficial in this form as pagemap
is binary in any case so you're not having to deal with overhead of parsing
a text file at least!
My thinking was, that if you have a large VMA, with ordinary pagemap you
have to copy 8byte per entry (and have room for that somewhere in user
space). In theory, with the scanning feature, you can leave that ...
scanning to the kernel and don't have to do any copying/allocate space
for it in user space etc.
--
Cheers,
David / dhildenb
