On Thu, 6 Apr 2023 at 19:58, Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx> wrote: > Hello, > > Thank you so much for the review. Do you have any thoughts on the build > error on arc architecture? > https://lore.kernel.org/all/e3c82373-256a-6297-bcb4-5e1179a2cbe2@xxxxxxxxxxxxx Maybe copy HPAGE_* defines from x86 and key on CONFIG_PGTABLE_LEVELS > 2? I don't know much about arc arch, though. > On 4/6/23 8:52 PM, Michał Mirosław wrote: > > On Thu, 6 Apr 2023 at 09:40, Muhammad Usama Anjum > > <usama.anjum@xxxxxxxxxxxxx> wrote:> [...] > >> +#define PM_SCAN_BITMAP(wt, file, present, swap) \ > >> + (wt | file << 1 | present << 2 | swap << 3) > > Please parenthesize macro arguments ("(wt)", "(file)", etc.) to not > > have to worry about operator precedence when passed a complex > > expression. > Like this? > #define PM_SCAN_BITMAP(wt, file, present, swap) \ > ((wt) | (file << 1) | (present << 2) | (swap << 3)) The value would be: ( (wt) | ((file) << 1) | ... ) IOW, each parameter should have parentheses around its name. [...] > >> + cur->len += n_pages; > >> + p->found_pages += n_pages; > >> + > >> + if (p->max_pages && (p->found_pages == p->max_pages)) > >> + return PM_SCAN_FOUND_MAX_PAGES; > >> + > >> + return 0; > >> + } > >> + > >> + if (!p->vec_index || ((p->vec_index + 1) < p->vec_len)) { > > > > It looks that `if (p->vec_index < p->vec_len)` is enough here - if we > > have vec_len == 0 here, then we'd not fit the entry in the userspace > > buffer anyway. Am I missing something? > No. I'd explained it with diagram last time: > https://lore.kernel.org/all/3c8d9ea0-1382-be0c-8dd2-d490eedd3b55@xxxxxxxxxxxxx > > I'll add a concise comment here. So it seems, but I think the code changed a bit and maybe could be simplified now? Since p->vec_len == 0 is currently not valid, the field could count only the entries available in p->vec[] -- IOW: not include p->cur in the count. BTW, `if (no space) return -ENOSPC` will avoid additional indentation for the non-merging case. [...] > >> +static inline int pagemap_scan_deposit(struct pagemap_scan_private *p, > >> + struct page_region __user *vec, > >> + unsigned long *vec_index) > > > > ..._deposit() is used only in single place - please inline. > It is already inline. Sorry. I mean: please paste the code in place of the single call. [...] > >> + /* > >> + * Break huge page into small pages if the WP operation need to > >> + * be performed is on a portion of the huge page or if max_pages > >> + * pages limit would exceed. > > > > BTW, could the `max_pages` limit be relaxed a bit (in that it would be > > possible to return more pages if they all merge into the last vector > > entry) so that it would not need to split otherwise-matching huge > > page? It would remove the need for this special handling in the kernel > > and splitting the page by this read-only-appearing ioctl? > No, this cannot be done. Otherwise we'll not be able to emulate Windows > syscall GetWriteWatch() which specifies the exact number of pages. Usually > in most of cases, either user will not use THP or not perform the operation > on partial huge page. So this part is only there to keep things correct for > those users who do use THP and partial pagemap_scan operations. I see that `GetWriteWatch` returns a list of pages not ranges of pages. That makes sense (more or less). (BTW, It could be emulated in userspace by caching the last not-fully-consumed range.) > >> + */ > >> + if (is_written && PM_SCAN_OP_IS_WP(p) && > >> + ((end - start < HPAGE_SIZE) || > >> + (p->max_pages && > >> + (p->max_pages - p->found_pages) < n_pages))) { > >> + > >> + split_huge_pmd(vma, pmd, start); > >> + goto process_smaller_pages; > >> + } > >> + > >> + if (p->max_pages && > >> + p->found_pages + n_pages > p->max_pages) > >> + n_pages = p->max_pages - p->found_pages; > >> + > >> + ret = pagemap_scan_output(is_written, is_file, is_present, > >> + is_swap, p, start, n_pages); > >> + if (ret < 0) > >> + return ret; So let's simplify this: if (p->max_pages && n_pages > max_pages - found_pages) n_pages = max_pages - found_pages; if (is_written && DO_WP && n_pages != HPAGE_SIZE / PAGE_SIZE) { split_thp(); goto process_smaller_pages; } BTW, THP handling could be extracted to a function that would return -EAGAIN if it has split the page or it wasn't a THP -- and that would mean `goto process_smaller_pages`. > > Why not propagate the error from uffd_wp_range()? > uffd_wp_range() returns status in long variable. We cannot return long in > this function. So intead of type casting long to int and then return I've > used -EINVAL. Would following be more suitable? > > long ret2 = uffd_wp_range(vma, start, HPAGE_SIZE, true); > if (ret2 < 0) > return (int)ret2; I think it's ok, since negative values are expected to be error codes. And since you can't overflow int with HPAGE_SIZE pages, then I wouldn't use `ret2` but cast the return and add a comment why it's safe. [...] > >> + start = (unsigned long)untagged_addr(arg.start); > >> + vec = (struct page_region *)(unsigned long)untagged_addr(arg.vec); > > > > Is the inner cast needed? > arg.vec remains 64-bit on 32-bit systems. So casting 64bit value directly > to struct page_region pointer errors out. So I've added specific casting to > unsigned long first before casting to pointers. I see. So to convey the intention, the `arg.start` and `arg.vec` should be casted to unsigned long, not the `untagged_addr()` return values. > >> + ret = pagemap_scan_args_valid(&arg, start, vec); > >> + if (ret) > >> + return ret; > >> + > >> + end = start + arg.len; > >> + p.max_pages = arg.max_pages; > >> + p.found_pages = 0; > >> + p.flags = arg.flags; > >> + p.required_mask = arg.required_mask; > >> + p.anyof_mask = arg.anyof_mask; > >> + p.excluded_mask = arg.excluded_mask; > >> + p.return_mask = arg.return_mask; > >> + p.cur.len = 0; > >> + p.cur.start = 0; > >> + p.vec = NULL; > >> + p.vec_len = (PAGEMAP_WALK_SIZE >> PAGE_SHIFT); > > > > Nit: parentheses are not needed here, please remove. > Will do. > > > > >> + > >> + /* > >> + * Allocate smaller buffer to get output from inside the page walk > >> + * functions and walk page range in PAGEMAP_WALK_SIZE size chunks. As > >> + * we want to return output to user in compact form where no two > >> + * consecutive regions should be continuous and have the same flags. > >> + * So store the latest element in p.cur between different walks and > >> + * store the p.cur at the end of the walk to the user buffer. > >> + */ > >> + p.vec = kmalloc_array(p.vec_len, sizeof(struct page_region), > >> + GFP_KERNEL); > >> + if (!p.vec) > >> + return -ENOMEM; > >> + > >> + walk_start = walk_end = start; > >> + while (walk_end < end && !ret) { > > > > The loop will stop if a previous iteration returned ENOSPC (and the > > error will be lost) - is it intended? > It is intentional. -ENOSPC means that the user buffer is full even though > there was more memory to walk over. We don't treat this error. So when > buffer gets full, we stop walking over further as user buffer has gotten > full and return as success. Thanks. What's the difference between -ENOSPC and PM_SCAN_FOUND_MAX_PAGES? They seem to result in the same effect (code flow). [...] > >> --- a/include/linux/userfaultfd_k.h > >> +++ b/include/linux/userfaultfd_k.h > >> @@ -210,6 +210,14 @@ extern bool userfaultfd_wp_async(struct vm_area_struct *vma); > >> > >> #else /* CONFIG_USERFAULTFD */ > >> > >> +static inline long uffd_wp_range(struct mm_struct *dst_mm, > >> + struct vm_area_struct *vma, > >> + unsigned long start, unsigned long len, > >> + bool enable_wp) > >> +{ > >> + return 0; > >> +} [...] > > Shouldn't this part be in the patch introducing uffd_wp_range()? > We have not added uffd_wp_range() in previous patches. We just need this > stub for this patch for the case when CONFIG_USERFAULTFD isn't enabled. > > I'd this as separate patch before this patch. Mike asked me to merge it > with this patch in order not to break bisectability. >[1] https://lore.kernel.org/all/ZBK+86eMMazwfhdx@xxxxxxxxxx I would understand the reply [1] to mean that the uffd_wp_range() stub should go in the same patch where uffd_wp_range() is implemented. But uffd_wp_range() is already in (since f369b07c86140) so I don't see how having the stub in a separate commit sequenced before this one could break bisect? Best Regards Michał Mirosław
