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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Oct 17, 2025 at 01:11:52PM -0700, Ackerley Tng wrote: > For shared to private conversions, if refcounts on any of the folios > within the range are elevated, fail the conversion with -EAGAIN. > > At the point of shared to private conversion, all folios in range are > also unmapped. The filemap_invalidate_lock() is held, so no faulting > can occur. Hence, from that point on, only transient refcounts can be > taken on the folios associated with that guest_memfd. > > Hence, it is safe to do the conversion from shared to private. > > After conversion is complete, refcounts may become elevated, but that > is fine since users of transient refcounts don't actually access > memory. > > For private to shared conversions, there are no refcount checks. any > transient refcounts are expected to drop their refcounts soon. The > conversion process will spin waiting for these transient refcounts to > go away. Where's the code to spin? > +/* > + * Preallocate memory for attributes to be stored on a maple tree, pointed to > + * by mas. Adjacent ranges with attributes identical to the new attributes > + * will be merged. Also sets mas's bounds up for storing attributes. > + * > + * This maintains the invariant that ranges with the same attributes will > + * always be merged. > + */ > +static int kvm_gmem_mas_preallocate(struct ma_state *mas, u64 attributes, > + pgoff_t start, size_t nr_pages) > +{ > + pgoff_t end = start + nr_pages; > + pgoff_t last = end - 1; > + void *entry; > + > + /* Try extending range. entry is NULL on overflow/wrap-around. */ > + mas_set_range(mas, end, end); > + entry = mas_find(mas, end); > + if (entry && xa_to_value(entry) == attributes) > + last = mas->last; > + > + mas_set_range(mas, start - 1, start - 1); Check start == 0 ? > + entry = mas_find(mas, start - 1); > + if (entry && xa_to_value(entry) == attributes) > + start = mas->index; > + > + mas_set_range(mas, start, last); > + return mas_preallocate(mas, xa_mk_value(attributes), GFP_KERNEL); > +} ... > +static long kvm_gmem_set_attributes(struct file *file, void __user *argp) > +{ > + struct gmem_file *f = file->private_data; > + struct inode *inode = file_inode(file); > + struct kvm_memory_attributes2 attrs; > + pgoff_t err_index; > + size_t nr_pages; > + pgoff_t index; > + int r; > + > + if (copy_from_user(&attrs, argp, sizeof(attrs))) > + return -EFAULT; > + > + if (attrs.flags) > + return -EINVAL; > + if (attrs.attributes & ~kvm_supported_mem_attributes(f->kvm)) > + return -EINVAL; > + if (attrs.size == 0 || attrs.offset + attrs.size < attrs.offset) > + return -EINVAL; > + if (!PAGE_ALIGNED(attrs.offset) || !PAGE_ALIGNED(attrs.offset)) Should be if (!PAGE_ALIGNED(attrs.offset) || !PAGE_ALIGNED(attrs.size)) ? > + return -EINVAL; > + > + if (attrs.offset > inode->i_size || Should be if (attrs.offset >= inode->i_size || ? > + attrs.offset + attrs.size > inode->i_size) > + return -EINVAL; > + > + nr_pages = attrs.size >> PAGE_SHIFT; > + index = attrs.offset >> PAGE_SHIFT; > + r = __kvm_gmem_set_attributes(inode, index, nr_pages, attrs.attributes, > + &err_index); > + if (r) { > + attrs.error_offset = err_index << PAGE_SHIFT; > + > + if (copy_to_user(argp, &attrs, sizeof(attrs))) > + return -EFAULT; > + } > + > + return r; > +}