Re: [PATCH] mm/madvise: preserve uprobe breakpoints across MADV_DONTNEED

["David Hildenbrand (Arm)" <david@kernel.org>]

On 4/29/26 15:15, Darko Tominac wrote:
> When uprobes are active, MADV_DONTNEED can discard file-backed pages
> that contain uprobe software breakpoint instructions.  Because the

If my memory serves me right, uprobes can only be installed in MAP_PRIVATE fil
mappings. Installing a uprobe breaks CoW by installing an anonymous page.

Not a file-backed page.

> uprobe infrastructure does not re-instrument pages on individual page
> faults (uprobe_mmap() is only called during VMA creation, not on
> page-in), the breakpoints are silently lost once the discarded pages are
> re-read from the backing file.  The probes stop firing with no error
> indication, and the only recovery is to unregister and re-register the
> affected uprobes.

Right. Don't MADV_DONTNEED uprobes, just like you are not supposed to
MADV_DONTNEED debugger breakpoints/set data etc. :)

> 
> Note that MADV_FREE is not affected: it only operates on anonymous VMAs
> (madvise_free_single_vma() rejects non-anonymous VMAs with -EINVAL),
> while uprobes only instrument file-backed mappings, so the two can never
> overlap.
> 
> A concrete example is a userspace memory reclamation subsystem that
> periodically calls madvise(MADV_DONTNEED) on file-backed text pages to
> release memory. 

It shouldn't do that on a MAP_PRIVATE file-backed VMA. It breaks the programn,
including uprobes and anything else that breaks CoW in there.

If it's using MADV_DONTNEED, it is damaging the application.

MADV_DONTNEED is not for memory reclaim.

-- 
Cheers,

David