Messages in this thread | | | Subject | Re: [PATCH RFC] mm/madvise: introduce MADV_POPULATE to prefault/prealloc memory | From | David Hildenbrand <> | Date | Fri, 19 Feb 2021 12:10:04 +0100 |
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On 19.02.21 12:04, Michal Hocko wrote: > On Fri 19-02-21 11:43:48, David Hildenbrand wrote: >> On 19.02.21 11:35, Michal Hocko wrote: >>> On Wed 17-02-21 16:48:44, David Hildenbrand wrote: >>> [...] >>> >>> I only got to the implementation now. >>> >>>> +static long madvise_populate(struct vm_area_struct *vma, >>>> + struct vm_area_struct **prev, >>>> + unsigned long start, unsigned long end) >>>> +{ >>>> + struct mm_struct *mm = vma->vm_mm; >>>> + unsigned long tmp_end; >>>> + int locked = 1; >>>> + long pages; >>>> + >>>> + *prev = vma; >>>> + >>>> + while (start < end) { >>>> + /* >>>> + * We might have temporarily dropped the lock. For example, >>>> + * our VMA might have been split. >>>> + */ >>>> + if (!vma || start >= vma->vm_end) { >>>> + vma = find_vma(mm, start); >>>> + if (!vma) >>>> + return -ENOMEM; >>>> + } >>> >>> Why do you need to find a vma when you already have one. do_madvise will >>> give you your vma already. I do understand that you want to finish the >>> vma for some errors but that shouldn't require handling vmas. You should >>> be in the shope of one here unless I miss anything. >> >> See below, we might temporary drop the lock while not having processed all >> pages >> >>> >>>> + >>>> + /* Bail out on incompatible VMA types. */ >>>> + if (vma->vm_flags & (VM_IO | VM_PFNMAP) || >>>> + !vma_is_accessible(vma)) { >>>> + return -EINVAL; >>>> + } >>>> + >>>> + /* >>>> + * Populate pages and take care of VM_LOCKED: simulate user >>>> + * space access. >>>> + * >>>> + * For private, writable mappings, trigger a write fault to >>>> + * break COW (i.e., shared zeropage). For other mappings (i.e., >>>> + * read-only, shared), trigger a read fault. >>>> + */ >>>> + tmp_end = min_t(unsigned long, end, vma->vm_end); >>>> + pages = populate_vma_page_range(vma, start, tmp_end, &locked); >>>> + if (!locked) { >>>> + mmap_read_lock(mm); >>>> + *prev = NULL; >>>> + vma = NULL; >> >> ^ here >> >> so, the VMA might have been replaced/split/... in the meantime. >> >> So to make forward progress, I have to lookup again. (similar. but different >> to madvise_dontneed_free()). > > Right. Missed that.
It would look more natural if we'd just be processing the whole range - but then it would not fit into the generic infrastructure and would result in even more code.
I decided to go with "process the passed range and treat the given VMA as an initial VMA that is invalidated as soon as we drop the lock".
-- Thanks,
David / dhildenb
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