On 15/05/2025 14:01, David Hildenbrand wrote:
On 15.05.25 12:07, Ryan Roberts wrote:
On 15/05/2025 09:53, 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:
I just reviewed the patch and had the same feedback as David. I agree with the patch below, with some small mods...
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));
You don't need both of these warnings; pmd_table() is only true if the pmd is present (well actually only if it's _valid_ which is more strict than present), so the second one is sufficient on its own.
Ah, right.
table = pte_offset_kernel(pmdp, addr); pmd_clear(pmdp); @@ -1305,7 +1303,8 @@ int pud_free_pmd_page(pud_t *pudp, unsigned long addr)
Given you are removing the runtime check and early return in pmd_free_pte_page(), I think you should modify this function to use the same style too.
BTW, the "return 1" is weird. But looking at x86, we seem to be making a private copy of the page table first, to defer freeing the page tables after the TLB flush.
I wonder if there isn't a better way (e.g., clear PUDP + flush tlb, then walk over the effectively-disconnected page table). But I'm sure there is a good reason for that.
As I understand it, the actual TLB entries should have been invalidated when the previous mappings we vfree'd. So the single page __flush_tlb_kernel_pgtable() calls here are to zap any table entries that may be in the walk cache. We could do an all-levels TLBI for the entire range, but for a system that doesn't support the tlbi-range operations, we would end up issuing a tlbi per page across the whole range which I think would be much slower than the one tlbi per pgtable we have here.
Things could be rearranged a bit so that we issue all the tlbis with only a single set of barriers (currently each __flush_tlb_kernel_pgtable() issues it's own barriers), but I'm not sure how important that micro-optimization is given I guess we never even call pud_free_pmd_page() in practice given we have had no reports of the warning tripping.
next = addr; end = addr + PUD_SIZE; do { - pmd_free_pte_page(pmdp, next); + if (pmd_present(*pmdp))
question: I wonder if it is better to use !pmd_none() as the condition here? It should either be none or a table at this point, so this allows the warning in pmd_free_pte_page() to catch more error conditions. No strong opinion though.
Same here. The existing callers check pmd_present().
Yeah fair let's be consistent and use pmd_present().
Thanks, Ryan