On Thu, May 15, 2025 at 03:04:50PM +0200, David Hildenbrand wrote:
On 15.05.25 14:56, Will Deacon wrote:
On Thu, May 15, 2025 at 11:32:22AM +0200, David Hildenbrand wrote:
On 15.05.25 11:27, Dev Jain wrote:
On 15/05/25 2:23 pm, David Hildenbrand wrote:
On 15.05.25 10:47, Dev Jain wrote:
On 15/05/25 2:06 pm, David Hildenbrand wrote: > On 15.05.25 10:22, Dev Jain wrote: > > > > > > On 15/05/25 1:43 pm, David Hildenbrand wrote: > > > On 15.05.25 08:34, Dev Jain wrote: > > > > Commit 9c006972c3fe removes the pxd_present() checks because the > > > > caller > > > > checks pxd_present(). But, in case of vmap_try_huge_pud(), the caller > > > > only > > > > checks pud_present(); pud_free_pmd_page() recurses on each pmd > > > > through > > > > pmd_free_pte_page(), wherein the pmd may be none. > > > The commit states: "The core code already has a check for pXd_none()", > > > so I assume that assumption was not true in all cases? > > > > > > Should that one problematic caller then check for pmd_none() instead? > > > > From what I could gather of Will's commit message, my > > interpretation is > > that the concerned callers are vmap_try_huge_pud and vmap_try_huge_pmd. > > These individually check for pxd_present(): > > > > if (pmd_present(*pmd) && !pmd_free_pte_page(pmd, addr)) > > return 0; > > > > The problem is that vmap_try_huge_pud will also iterate on pte entries. > > So if the pud is present, then pud_free_pmd_page -> pmd_free_pte_page > > may encounter a none pmd and trigger a WARN. > > Yeah, pud_free_pmd_page()->pmd_free_pte_page() looks shaky. > > I assume we should either have an explicit pmd_none() check in > pud_free_pmd_page() before calling pmd_free_pte_page(), or one in > pmd_free_pte_page(). > > With your patch, we'd be calling pte_free_kernel() on a NULL pointer, > which sounds wrong -- unless I am missing something important.
Ah thanks, you seem to be right. We will be extracting table from a none pmd. Perhaps we should still bail out for !pxd_present() but without the warning, which the fix commit used to do.
Right. We just make sure that all callers of pmd_free_pte_page() already check for it.
I'd just do something like:
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index 8fcf59ba39db7..e98dd7af147d5 100644 --- a/arch/arm64/mm/mmu.c +++ b/arch/arm64/mm/mmu.c @@ -1274,10 +1274,8 @@ int pmd_free_pte_page(pmd_t *pmdp, unsigned long addr)
pmd = READ_ONCE(*pmdp);
- if (!pmd_table(pmd)) { - VM_WARN_ON(1); - return 1; - } + VM_WARN_ON(!pmd_present(pmd)); + VM_WARN_ON(!pmd_table(pmd));
And also return 1?
I'll leave that to Catalin + Will.
I'm not a friend for adding runtime-overhead for soemthing that should not happen and be caught early during testing -> VM_WARN_ON_ONCE().
I definitely think we should return early if the pmd isn't a table. Otherwise, we could end up descending into God-knows-what!
The question is: how did something that is not a table end up here, and why is it valid to check exactly that at runtime. Not strong opinion, it just feels a bit arbitrary to test for exactly that at runtime if it is completely unexpected.
I see it a little bit like type-checking: we could see an invalid entry, a leaf entry or a table entry and we should only ever dereference the latter. If the VM_WARN_ON() is justified, then I find it jarring to go ahead with the dereference regardless of the type.
That said, maybe the VM_WARN_ON() should either be deleted or moved out to the callers in mm/vmalloc.c?
Will