Re: [RFC/PATCH 01/22] s390/mm: make gmap_protect_range more modular

David Hildenbrand
2017-11-08T10:40:22Z

On 06.11.2017 23:29, Janosch Frank wrote:
> This patch reworks the gmap_protect_range logic and extracts the pte
> handling into an own function. Also we do now walk to the pmd and make
> it accessible in the function for later use. This way we can add huge
> page handling logic more easily.

I just realized (and hope it is correct), that any gmap_shadow() checks
in e.g. gmap_pte_op_walk() are superfluous. This code is never reached.

This would imply that also in this patch, you can drop all
gmap_is_shadow(gmap) checks and instead add BUG_ON(gmap_is_shadow()) to
all functions.

Will double check and prepare a cleanup for existing code.

>
> Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
> Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
> Reviewed-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
> ---
> arch/s390/mm/gmap.c | 104 +++++++++++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 95 insertions(+), 9 deletions(-)
>
> diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
> index 2f66290..9757242 100644
> --- a/arch/s390/mm/gmap.c
> +++ b/arch/s390/mm/gmap.c
> @@ -876,6 +876,91 @@ static void gmap_pte_op_end(spinlock_t *ptl)
> spin_unlock(ptl);
> }
>
> +/**
> + * gmap_pmd_op_walk - walk the gmap tables, get the guest table lock
> + * and return the pmd pointer
> + * @gmap: pointer to guest mapping meta data structure
> + * @gaddr: virtual address in the guest address space
> + *
> + * Returns a pointer to the pmd for a guest address, or NULL
> + */
> +static inline pmd_t *gmap_pmd_op_walk(struct gmap *gmap, unsigned long gaddr)
> +{
> + pmd_t *pmdp;
> +
> + spin_lock(&gmap->guest_table_lock);
> + pmdp = (pmd_t *) gmap_table_walk(gmap, gaddr, 1);
> +
> + /*
> + * Empty pmds can become large after we give up the
> + * guest_table_lock, so we have to check for pmd_none
> + * here.
> + */
> + if (!pmdp || pmd_none(*pmdp)) {
> + spin_unlock(&gmap->guest_table_lock);
> + return NULL;
> + }
> + /*
> + * For plain 4k guests that do not run under the vsie it
> + * suffices to take the pte lock later on. Thus we can unlock
> + * the guest_table_lock here.
> + */
> + if (!pmd_large(*pmdp) && !gmap_is_shadow(gmap))
> + spin_unlock(&gmap->guest_table_lock);
> + return pmdp;
> +}
> +
> +/**
> + * gmap_pmd_op_end - release the guest_table_lock if needed
> + * @gmap: pointer to the guest mapping meta data structure
> + * @pmdp: pointer to the pmd
> + */
> +static inline void gmap_pmd_op_end(struct gmap *gmap, pmd_t *pmdp)
> +{
> + if (pmd_large(*pmdp) || gmap_is_shadow(gmap))
> + spin_unlock(&gmap->guest_table_lock);
> +}
> +
> +/*
> + * gmap_protect_pte - remove access rights to memory and set pgste bits
> + * @gmap: pointer to guest mapping meta data structure
> + * @gaddr: virtual address in the guest address space
> + * @pmdp: pointer to the pmd associated with the pte
> + * @prot: indicates access rights: PROT_NONE, PROT_READ or PROT_WRITE
> + * @bits: pgste notification bits to set
> + *
> + * Returns 0 if successfully protected, -ENOMEM if out of memory and
> + * -EAGAIN if a fixup is needed.
> + *
> + * Expected to be called with sg->mm->mmap_sem in read and
> + * guest_table_lock held for shadow gmaps.
> + */
> +static int gmap_protect_pte(struct gmap *gmap, unsigned long gaddr,
> + pmd_t *pmdp, int prot, unsigned long bits)
> +{
> + int rc;
> + pte_t *ptep;
> + spinlock_t *ptl = NULL;
> +
> + /* We have no upper segment, let's go back and fix this up. */
> + if (pmd_val(*pmdp) & _SEGMENT_ENTRY_INVALID)
> + return -EAGAIN;
> +
> + if (gmap_is_shadow(gmap)) {
> + ptep = pte_offset_map(pmdp, gaddr);
> + } else {
> + ptep = pte_alloc_map_lock(gmap->mm, pmdp, gaddr, &ptl);
> + if (!ptep)
> + return -ENOMEM;
> + }

if (gmap_is_shadow(gmap))
ptep = pte_offset_map(pmdp, gaddr);
else
ptep = pte_alloc_map_lock(gmap->mm, pmdp, gaddr, &ptl);

if (!ptep)
return -ENOMEM;

Makes it a little bit nicer to read.

> +
> + /* Protect and unlock. */
> + rc = ptep_force_prot(gmap->mm, gaddr, ptep, prot, bits);
> + if (ptl)
> + gmap_pte_op_end(ptl);

Would it make sense to move the ptl test into gmap_pte_op_end() ?

> + return rc;
> +}
> +
> /*
> * gmap_protect_range - remove access rights to memory and set pgste bits
> * @gmap: pointer to guest mapping meta data structure
> @@ -895,16 +980,20 @@ static int gmap_protect_range(struct gmap *gmap, unsigned long gaddr,
> unsigned long len, int prot, unsigned long bits)
> {
> unsigned long vmaddr;
> - spinlock_t *ptl;
> - pte_t *ptep;
> + pmd_t *pmdp;
> int rc;
>
> while (len) {
> rc = -EAGAIN;
> - ptep = gmap_pte_op_walk(gmap, gaddr, &ptl);
> - if (ptep) {
> - rc = ptep_force_prot(gmap->mm, gaddr, ptep, prot, bits);
> - gmap_pte_op_end(ptl);
> + pmdp = gmap_pmd_op_walk(gmap, gaddr);
> + if (pmdp) {
> + rc = gmap_protect_pte(gmap, gaddr, pmdp, prot,
> + bits);
> + if (!rc) {
> + len -= PAGE_SIZE;
> + gaddr += PAGE_SIZE;
> + }
> + gmap_pmd_op_end(gmap, pmdp);
> }
> if (rc) {
> vmaddr = __gmap_translate(gmap, gaddr);
> @@ -913,10 +1002,7 @@ static int gmap_protect_range(struct gmap *gmap, unsigned long gaddr,
> rc = gmap_pte_op_fixup(gmap, gaddr, vmaddr, prot);
> if (rc)
> return rc;
> - continue;
> }
> - gaddr += PAGE_SIZE;
> - len -= PAGE_SIZE;

Not sure if I like this change. I think I prefer it the way it is now
(keeps the loop body shorter).

> }
> return 0;
> }
>

--

Thanks,

David / dhildenb