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 eNEDtMMg 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000007, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Mar 07, 2025 at 01:42:13PM +0000, Ryan Roberts wrote: > On 07/03/2025 13:04, Lorenzo Stoakes wrote: > > On Fri, Mar 07, 2025 at 12:33:06PM +0000, Ryan Roberts wrote: > >> Instead of writing a pte directly into the table, use the set_pte_at() > >> helper, which gives the arch visibility of the change. > >> > >> In this instance we are guaranteed that the pte was originally none and > >> is being modified to a not-present pte, so there was unlikely to be a > >> bug in practice (at least not on arm64). But it's bad practice to write > >> the page table memory directly without arch involvement. > >> > >> Cc: > >> Fixes: 662df3e5c376 ("mm: madvise: implement lightweight guard page mechanism") > >> Signed-off-by: Ryan Roberts > >> --- > >> mm/madvise.c | 2 +- > >> 1 file changed, 1 insertion(+), 1 deletion(-) > >> > >> diff --git a/mm/madvise.c b/mm/madvise.c > >> index 388dc289b5d1..6170f4acc14f 100644 > >> --- a/mm/madvise.c > >> +++ b/mm/madvise.c > >> @@ -1101,7 +1101,7 @@ static int guard_install_set_pte(unsigned long addr, unsigned long next, > >> unsigned long *nr_pages = (unsigned long *)walk->private; > >> > >> /* Simply install a PTE marker, this causes segfault on access. */ > >> - *ptep = make_pte_marker(PTE_MARKER_GUARD); > >> + set_pte_at(walk->mm, addr, ptep, make_pte_marker(PTE_MARKER_GUARD)); > > > > I agree with you, but I think perhaps the arg name here is misleading :) If > > you look at mm/pagewalk.c and specifically, in walk_pte_range_inner(): > > > > if (ops->install_pte && pte_none(ptep_get(pte))) { > > pte_t new_pte; > > > > err = ops->install_pte(addr, addr + PAGE_SIZE, &new_pte, > > walk); > > if (err) > > break; > > > > set_pte_at(walk->mm, addr, pte, new_pte); > > > > ... > > } > > > > So the ptep being assigned here is a stack value, new_pte, which we simply > > assign to, and _then_ the page walker code handles the set_pte_at() for us. > > > > So we are indeed doing the right thing here, just in a different place :P > > Ahh my bad. In that case, please ignore the patch. > > But out of interest, why are you doing it like this? I find it a bit confusing > as all the other ops (e.g. pte_entry()) work directly on the pgtable's pte > without the intermediate. In those cases it's read-only, the data's already there, you can just go ahead and manipulate it (and would expect to be able to do so). When setting things are a little different, I'd rather not open up things to a user being able to do *whatever*, but rather limit to the smallest scope possible for installing the PTE. And also of course, it allows us to _mandate_ that set_pte_at() is used so we do the right thing re: arches :) I could have named the parameter better though, in guard_install_pte_entry() would be better to have called it 'new_pte' or something. > > Thanks, > Ryan > > > > >> (*nr_pages)++; > >> > >> return 0; > >> -- > >> 2.43.0 > >> > Thanks for looking at this by the way, obviously I appreciate your point in chasing up cases like this as endeavoured to do the right thing here, albeit abstracted away :)