>
> Let's cc Uladzislau on vmalloc things?
>
> > How about this?
>
> Well, lol, that's a simple approach to avoiding the problem ;)
>
To me it looks like a specific workaround for a specific one user.
> > unmap_kernel_range had been atomic operation and zsmalloc has used
> > it in atomic context in zs_unmap_object.
> > However, ("e47110e90584, mm/vunmap: add cond_resched() in vunmap_pmd_range")
> > changed it into non-atomic operation via adding cond_resched.
> > It causes zram decompresion failure by corrupting compressed buffer
> > in atomic context.
> >
> > This patch introduces unmap_kernel_range_atomic which works for
> > only range less than PMD_SIZE to prevent cond_resched call.
> >
> > ...
> >
> > --- a/include/linux/vmalloc.h
> > +++ b/include/linux/vmalloc.h
> > @@ -180,6 +180,7 @@ int map_kernel_range(unsigned long start, unsigned long size, pgprot_t prot,
> > struct page **pages);
> > extern void unmap_kernel_range_noflush(unsigned long addr, unsigned long size);
> > extern void unmap_kernel_range(unsigned long addr, unsigned long size);
> > +extern void unmap_kernel_range_atomic(unsigned long addr, unsigned long size);
> > static inline void set_vm_flush_reset_perms(void *addr)
> > {
> > struct vm_struct *vm = find_vm_area(addr);
> > @@ -200,6 +201,7 @@ unmap_kernel_range_noflush(unsigned long addr, unsigned long size)
> > {
> > }
> > #define unmap_kernel_range unmap_kernel_range_noflush
> > +#define unmap_kernel_range_atomic unmap_kernel_range_noflush
> > static inline void set_vm_flush_reset_perms(void *addr)
> > {
> > }
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index d7075ad340aa..714e5425dc45 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -88,6 +88,7 @@ static void vunmap_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
> > pmd_t *pmd;
> > unsigned long next;
> > int cleared;
> > + bool check_resched = (end - addr) > PMD_SIZE;
> >
> > pmd = pmd_offset(pud, addr);
> > do {
> > @@ -102,8 +103,8 @@ static void vunmap_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
> > if (pmd_none_or_clear_bad(pmd))
> > continue;
> > vunmap_pte_range(pmd, addr, next, mask);
> > -
> > - cond_resched();
> > + if (check_resched)
> > + cond_resched();
> > } while (pmd++, addr = next, addr != end);
> > }
> >
> > @@ -2024,6 +2025,24 @@ void unmap_kernel_range(unsigned long addr, unsigned long size)
> > flush_tlb_kernel_range(addr, end);
> > }
> >
> > +/**
> > + * unmap_kernel_range_atomic - unmap kernel VM area and flush cache and TLB
> > + * @addr: start of the VM area to unmap
> > + * @size: size of the VM area to unmap
> > + *
> > + * Similar to unmap_kernel_range_noflush() but it's atomic. @size should be
> > + * less than PMD_SIZE.
> > + */
> > +void unmap_kernel_range_atomic(unsigned long addr, unsigned long size)
> > +{
> > + unsigned long end = addr + size;
> > +
> > + flush_cache_vunmap(addr, end);
> > + WARN_ON(size > PMD_SIZE);
>
> WARN_ON_ONCE() would be better here - no point in creating a million
> warnings where one would suffice.
>
> > + unmap_kernel_range_noflush(addr, size);
> > + flush_tlb_kernel_range(addr, end);
> > +}
> > +
> > static inline void setup_vmalloc_vm_locked(struct vm_struct *vm,
> > struct vmap_area *va, unsigned long flags, const void *caller)
> > {
> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > index 662ee420706f..9decc7634852 100644
> > --- a/mm/zsmalloc.c
> > +++ b/mm/zsmalloc.c
> > @@ -1154,7 +1154,7 @@ static inline void __zs_unmap_object(struct mapping_area *area,
> > {
> > unsigned long addr = (unsigned long)area->vm_addr;
> >
> > - unmap_kernel_range(addr, PAGE_SIZE * 2);
> > + unmap_kernel_range_atomic(addr, PAGE_SIZE * 2);
> > }
>
> I suppose we could live with it if no better solutions are forthcoming.
>
Maybe solve it on zsmalloc side? For example to add __zs_unmap_object_deferred(),
so it schedules the work that calls unmap_kernel_range() on a list of mapping_area
objects.
--
Vlad Rezki
