linux-kernel - Re: [RFC][PATCH] uprobe: support for private hugetlb mappings

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Message-ID: <Zip0fEliGeL0qmID@bender.morinfr.org>
Date: Thu, 25 Apr 2024 17:19:24 +0200
From: Guillaume Morin <guillaume@...infr.org>
To: David Hildenbrand <david@...hat.com>
Cc: Guillaume Morin <guillaume@...infr.org>, oleg@...hat.com,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	muchun.song@...ux.dev
Subject: Re: [RFC][PATCH] uprobe: support for private hugetlb mappings

On 24 Apr 23:00, David Hildenbrand wrote:
> > One issue here is that FOLL_FORCE|FOLL_WRITE is not implemented for
> > hugetlb mappings. However this was also on my TODO and I have a draft
> > patch that implements it.
> 
> Yes, I documented it back then and added sanity checks in GUP code to fence
> it off. Shouldn't be too hard to implement (famous last words) and would be
> the cleaner thing to use here once I manage to switch over to
> FOLL_WRITE|FOLL_FORCE to break COW.

Yes, my patch seems to be working. The hugetlb code is pretty simple.
And it allows ptrace and the proc pid mem file to work on the executable
private hugetlb mappings.

There is one thing I am unclear about though. hugetlb enforces that
huge_pte_write() is true on FOLL_WRITE in both the fault and
follow_page_mask paths. I am not sure if we can simply assume in the
hugetlb code that if the pte is not writable and this is a write fault
then we're in the FOLL_FORCE|FOLL_WRITE case.  Or do we want to keep the
checks simply not enforce it for FOLL_FORCE|FOLL_WRITE?

The latter is more complicated in the fault path because there is no
FAULT_FLAG_FORCE flag.

-- 
Guillaume Morin <guillaume@...infr.org>