Re: [PATCH v5 05/26] mm/swap: Introduce the idea of special swap ptes

Alistair Popple
2021-07-16T05:51:26Z

Hi Peter,

[...]

> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index ae1f5d0cb581..4b46c099ad94 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5738,7 +5738,7 @@ static enum mc_target_type get_mctgt_type(struct vm_area_struct *vma,
>
> if (pte_present(ptent))
> page = mc_handle_present_pte(vma, addr, ptent);
> - else if (is_swap_pte(ptent))
> + else if (pte_has_swap_entry(ptent))
> page = mc_handle_swap_pte(vma, ptent, &ent);
> else if (pte_none(ptent))
> page = mc_handle_file_pte(vma, addr, ptent, &ent);

As I understand things pte_none() == False for a special swap pte, but
shouldn't this be treated as pte_none() here? Ie. does this need to be
pte_none(ptent) || is_swap_special_pte() here?

> diff --git a/mm/memory.c b/mm/memory.c
> index 0e0de08a2cd5..998a4f9a3744 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -3491,6 +3491,13 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> if (!pte_unmap_same(vmf))
> goto out;
>
> + /*
> + * We should never call do_swap_page upon a swap special pte; just be
> + * safe to bail out if it happens.
> + */
> + if (WARN_ON_ONCE(is_swap_special_pte(vmf->orig_pte)))
> + goto out;
> +
> entry = pte_to_swp_entry(vmf->orig_pte);
> if (unlikely(non_swap_entry(entry))) {
> if (is_migration_entry(entry)) {

Are there other changes required here? Because we can end up with stale special
pte's and a special pte is !pte_none don't we need to fix some of the !pte_none
checks in these functions:

insert_pfn() -> checks for !pte_none
remap_pte_range() -> BUG_ON(!pte_none)
apply_to_pte_range() -> didn't check further but it tests for !pte_none

In general it feels like I might be missing something here though. There are
plenty of checks in the kernel for pte_none() which haven't been updated. Is
there some rule that says none of those paths can see a special pte?

> diff --git a/mm/migrate.c b/mm/migrate.c
> index 23cbd9de030b..b477d0d5f911 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -294,7 +294,7 @@ void __migration_entry_wait(struct mm_struct *mm, pte_t *ptep,
>
> spin_lock(ptl);
> pte = *ptep;
> - if (!is_swap_pte(pte))
> + if (!pte_has_swap_entry(pte))
> goto out;
>
> entry = pte_to_swp_entry(pte);
> @@ -2276,7 +2276,7 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>
> pte = *ptep;
>
> - if (pte_none(pte)) {
> + if (pte_none(pte) || is_swap_special_pte(pte)) {

I was wondering if we can loose the special pte information here? However I see
that in migrate_vma_insert_page() we check again and fail the migration if
!pte_none() so I think this is ok.

I think it would be better if this check was moved below so the migration fails
early. Ie:

if (pte_none(pte)) {
if (vma_is_anonymous(vma) && !is_swap_special_pte(pte)) {

Also how does this work for page migration in general? I can see in
page_vma_mapped_walk() that we skip special pte's, but doesn't this mean we
loose the special pte in that instance? Or is that ok for some reason?

> if (vma_is_anonymous(vma)) {
> mpfn = MIGRATE_PFN_MIGRATE;
> migrate->cpages++;
> diff --git a/mm/mincore.c b/mm/mincore.c
> index 9122676b54d6..5728c3e6473f 100644
> --- a/mm/mincore.c
> +++ b/mm/mincore.c
> @@ -121,7 +121,7 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> for (; addr != end; ptep++, addr += PAGE_SIZE) {
> pte_t pte = *ptep;
>
> - if (pte_none(pte))
> + if (pte_none(pte) || is_swap_special_pte(pte))
> __mincore_unmapped_range(addr, addr + PAGE_SIZE,
> vma, vec);
> else if (pte_present(pte))
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 883e2cc85cad..4b743394afbe 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -139,7 +139,7 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> }
> ptep_modify_prot_commit(vma, addr, pte, oldpte, ptent);
> pages++;
> - } else if (is_swap_pte(oldpte)) {
> + } else if (pte_has_swap_entry(oldpte)) {
> swp_entry_t entry = pte_to_swp_entry(oldpte);
> pte_t newpte;
>
> diff --git a/mm/mremap.c b/mm/mremap.c
> index 5989d3990020..122b279333ee 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -125,7 +125,7 @@ static pte_t move_soft_dirty_pte(pte_t pte)
> #ifdef CONFIG_MEM_SOFT_DIRTY
> if (pte_present(pte))
> pte = pte_mksoft_dirty(pte);
> - else if (is_swap_pte(pte))
> + else if (pte_has_swap_entry(pte))
> pte = pte_swp_mksoft_dirty(pte);
> #endif
> return pte;
> diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c
> index f7b331081791..ff57b67426af 100644
> --- a/mm/page_vma_mapped.c
> +++ b/mm/page_vma_mapped.c
> @@ -36,7 +36,7 @@ static bool map_pte(struct page_vma_mapped_walk *pvmw)
> * For more details on device private memory see HMM
> * (include/linux/hmm.h or mm/hmm.c).
> */
> - if (is_swap_pte(*pvmw->pte)) {
> + if (pte_has_swap_entry(*pvmw->pte)) {
> swp_entry_t entry;
>
> /* Handle un-addressable ZONE_DEVICE memory */
> @@ -90,7 +90,7 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
>
> if (pvmw->flags & PVMW_MIGRATION) {
> swp_entry_t entry;
> - if (!is_swap_pte(*pvmw->pte))
> + if (!pte_has_swap_entry(*pvmw->pte))
> return false;
> entry = pte_to_swp_entry(*pvmw->pte);
>
> @@ -99,7 +99,7 @@ static bool check_pte(struct page_vma_mapped_walk *pvmw)
> return false;
>
> pfn = swp_offset(entry);
> - } else if (is_swap_pte(*pvmw->pte)) {
> + } else if (pte_has_swap_entry(*pvmw->pte)) {
> swp_entry_t entry;
>
> /* Handle un-addressable ZONE_DEVICE memory */
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 1e07d1c776f2..4993b4454c13 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1951,7 +1951,7 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> si = swap_info[type];
> pte = pte_offset_map(pmd, addr);
> do {
> - if (!is_swap_pte(*pte))
> + if (!pte_has_swap_entry(*pte))
> continue;
>
> entry = pte_to_swp_entry(*pte);
>