"Liam R. Howlett" <[email protected]> writes:
> On 26/04/28 04:25PM, Ackerley Tng via B4 Relay wrote:
>>
>> [...snip...]
>>
>> +/*
>> + * 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);
Thank you for your reviews!
>
> Please read the documentation as I believe you have a bug here. What
> happens if there is another range stored higher than end + 1?
>
The invariant in this maple tree is that contiguous ranges with the same
attribute are stored as a single range.
The goal of this first part is to get the entry at the index just after
the requested range, and see what the attribute there is. If that
attribute is what we're about to set, extend the requested range for
storing to the end of that range.
If there is another range higher than end + 1, with the invariant
maintained, that attribute has to be different than the attribute stored
at end. Hence, we only want to extend this requested range up till end.
> Do you have testing of these functions somewhere?
>
GMEM_CONVERSION_MULTIPAGE_TEST_INIT_SHARED(indexing, 4) tests setting
attributes in ranges. If test_page is 2,
1. [0, 4) starts off shared (4 is the number of pages in the guest_memfd)
2. [2, 3) is converted to private
=> so the ranges should now be [0, 2), [2, 3), [3, 4)
3. [2, 3) is converted back to shared
=> so the ranges should now be [0, 4)
I verified this by inserting some trace_printk()s and inspecting manually.
>> + if (entry && xa_to_value(entry) == attributes)
>> + last = mas->last;
>> +
>> + if (start > 0) {
>> + mas_set_range(mas, start - 1, start - 1);
>> + 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);
>> +}
>> +
>>
>> [...snip...]
>>
