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    SubjectRe: [RFC PATCH 11/15] memremap: Add zone device access protection
    On Tue, Jul 14, 2020 at 12:02:16AM -0700, ira.weiny@intel.com wrote:

    > +static pgprot_t dev_protection_enable_get(struct dev_pagemap *pgmap, pgprot_t prot)
    > +{
    > + if (pgmap->flags & PGMAP_PROT_ENABLED && dev_page_pkey != PKEY_INVALID) {
    > + pgprotval_t val = pgprot_val(prot);
    > +
    > + mutex_lock(&dev_prot_enable_lock);
    > + dev_protection_enable++;
    > + /* Only enable the static branch 1 time */
    > + if (dev_protection_enable == 1)
    > + static_branch_enable(&dev_protection_static_key);
    > + mutex_unlock(&dev_prot_enable_lock);
    > +
    > + prot = __pgprot(val | _PAGE_PKEY(dev_page_pkey));
    > + }
    > + return prot;
    > +}
    > +
    > +static void dev_protection_enable_put(struct dev_pagemap *pgmap)
    > +{
    > + if (pgmap->flags & PGMAP_PROT_ENABLED && dev_page_pkey != PKEY_INVALID) {
    > + mutex_lock(&dev_prot_enable_lock);
    > + dev_protection_enable--;
    > + if (dev_protection_enable == 0)
    > + static_branch_disable(&dev_protection_static_key);
    > + mutex_unlock(&dev_prot_enable_lock);
    > + }
    > +}

    That's an anti-pattern vs static_keys, I'm thinking you actually want
    static_key_slow_{inc,dec}() instead of {enable,disable}().

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