Messages in this thread | | | Subject | Re: [RFC v6 PATCH 1/2] mm: refactor do_munmap() to extract the common part | From | Yang Shi <> | Date | Fri, 3 Aug 2018 13:47:19 -0700 |
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On 8/3/18 1:53 AM, Michal Hocko wrote: > On Fri 27-07-18 02:10:13, Yang Shi wrote: >> Introduces three new helper functions: >> * munmap_addr_sanity() >> * munmap_lookup_vma() >> * munmap_mlock_vma() >> >> They will be used by do_munmap() and the new do_munmap with zapping >> large mapping early in the later patch. >> >> There is no functional change, just code refactor. >> >> Reviewed-by: Laurent Dufour <ldufour@linux.vnet.ibm.com> >> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com> >> --- >> mm/mmap.c | 120 ++++++++++++++++++++++++++++++++++++++++++-------------------- >> 1 file changed, 82 insertions(+), 38 deletions(-) >> >> diff --git a/mm/mmap.c b/mm/mmap.c >> index d1eb87e..2504094 100644 >> --- a/mm/mmap.c >> +++ b/mm/mmap.c >> @@ -2686,34 +2686,44 @@ int split_vma(struct mm_struct *mm, struct vm_area_struct *vma, >> return __split_vma(mm, vma, addr, new_below); >> } >> >> -/* Munmap is split into 2 main parts -- this part which finds >> - * what needs doing, and the areas themselves, which do the >> - * work. This now handles partial unmappings. >> - * Jeremy Fitzhardinge <jeremy@goop.org> >> - */ >> -int do_munmap(struct mm_struct *mm, unsigned long start, size_t len, >> - struct list_head *uf) >> +static inline bool munmap_addr_sanity(unsigned long start, size_t len) > munmap_check_addr? Btw. why does this need to have munmap prefix at all? > This is a general address space check.
Just because I extracted this from do_munmap, no special consideration. It is definitely ok to use another name.
> >> { >> - unsigned long end; >> - struct vm_area_struct *vma, *prev, *last; >> - >> if ((offset_in_page(start)) || start > TASK_SIZE || len > TASK_SIZE-start) >> - return -EINVAL; >> + return false; >> >> - len = PAGE_ALIGN(len); >> - if (len == 0) >> - return -EINVAL; >> + if (PAGE_ALIGN(len) == 0) >> + return false; >> + >> + return true; >> +} >> + >> +/* >> + * munmap_lookup_vma: find the first overlap vma and split overlap vmas. >> + * @mm: mm_struct >> + * @vma: the first overlapping vma >> + * @prev: vma's prev >> + * @start: start address >> + * @end: end address > This really doesn't help me to understand how to use the function. > Why do we need both prev and vma etc...
prev will be used by unmap_region later.
> >> + * >> + * returns 1 if successful, 0 or errno otherwise > This is a really weird calling convention. So what does 0 tell? /me > checks the code. Ohh, it is nothing to do. Why cannot you simply return > the vma. NULL implies nothing to do, ERR_PTR on error.
A couple of reasons why it is implemented as so:
* do_munmap returns 0 for both success and no suitable vma
* Since prev is needed by finding the start vma, and prev will be used by unmap_region later too, so I just thought it would look clean to have one function to return both start vma and prev. In this way, we can share as much as possible common code.
* In this way, we just need return 0, 1 or error no just as same as what do_munmap does currently. Then we know what is failure case exactly to just bail out right away.
Actually, I tried the same approach as you suggested, but it had two problems:
* If it returns the start vma, we have to re-find its prev later, but the prev has been found during finding start vma. And, duplicate the code in do_munmap_zap_rlock. It sounds not that ideal.
* If it returns prev, it might be null (start vma is the first vma). We can't tell if null is a failure or success case
> >> + */ >> +static int munmap_lookup_vma(struct mm_struct *mm, struct vm_area_struct **vma, >> + struct vm_area_struct **prev, unsigned long start, >> + unsigned long end) >> +{ >> + struct vm_area_struct *tmp, *last; >> >> /* Find the first overlapping VMA */ >> - vma = find_vma(mm, start); >> - if (!vma) >> + tmp = find_vma(mm, start); >> + if (!tmp) >> return 0; >> - prev = vma->vm_prev; >> - /* we have start < vma->vm_end */ >> + >> + *prev = tmp->vm_prev; > Why do you set prev here. We might "fail" with 0 right after this
No special reason, just copied from do_munmap. Yes, it is ideal to have prev set here. It can be moved further down.
> >> + >> + /* we have start < vma->vm_end */ >> >> /* if it doesn't overlap, we have nothing.. */ >> - end = start + len; >> - if (vma->vm_start >= end) >> + if (tmp->vm_start >= end) >> return 0; >> >> /* >> @@ -2723,7 +2733,7 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len, >> * unmapped vm_area_struct will remain in use: so lower split_vma >> * places tmp vma above, and higher split_vma places tmp vma below. >> */ >> - if (start > vma->vm_start) { >> + if (start > tmp->vm_start) { >> int error; >> >> /* >> @@ -2731,13 +2741,14 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len, >> * not exceed its limit; but let map_count go just above >> * its limit temporarily, to help free resources as expected. >> */ >> - if (end < vma->vm_end && mm->map_count >= sysctl_max_map_count) >> + if (end < tmp->vm_end && >> + mm->map_count > sysctl_max_map_count) >> return -ENOMEM; >> >> - error = __split_vma(mm, vma, start, 0); >> + error = __split_vma(mm, tmp, start, 0); >> if (error) >> return error; >> - prev = vma; >> + *prev = tmp; >> } >> >> /* Does it split the last one? */ >> @@ -2747,7 +2758,48 @@ int do_munmap(struct mm_struct *mm, unsigned long start, size_t len, >> if (error) >> return error; >> } >> - vma = prev ? prev->vm_next : mm->mmap; >> + >> + *vma = *prev ? (*prev)->vm_next : mm->mmap; >> + >> + return 1; >> +} > the patch would be much more easier to read if you didn't do vma->tmp > renaming.
Yes, I should used another name for the "vma" argument.
Thanks, Yang
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