Re: [PATCH 17/19] powerpc: book3s64: convert to pin_user_pages() and put_user_page()

John Hubbard
2019-11-25T20:47:06Z

On 11/25/19 12:59 AM, Jan Kara wrote:
> On Sun 24-11-19 20:20:09, John Hubbard wrote:
>> 1. Convert from get_user_pages() to pin_user_pages().
>>
>> 2. As required by pin_user_pages(), release these pages via
>> put_user_page(). In this case, do so via put_user_pages_dirty_lock().
>>
>> That has the side effect of calling set_page_dirty_lock(), instead
>> of set_page_dirty(). This is probably more accurate.
>>
>> As Christoph Hellwig put it, "set_page_dirty() is only safe if we are
>> dealing with a file backed page where we have reference on the inode it
>> hangs off." [1]
>>
>> 3. Release each page in mem->hpages[] (instead of mem->hpas[]), because
>> that is the array that pin_longterm_pages() filled in. This is more
>> accurate and should be a little safer from a maintenance point of
>> view.
>
> Except that this breaks the code. hpages is unioned with hpas...
>

OK.

>> @@ -212,10 +211,9 @@ static void mm_iommu_unpin(struct mm_iommu_table_group_mem_t *mem)
>> if (!page)
>> continue;
>>
>> - if (mem->hpas[i] & MM_IOMMU_TABLE_GROUP_PAGE_DIRTY)
>> - SetPageDirty(page);
>> + put_user_pages_dirty_lock(&mem->hpages[i], 1,
>> + MM_IOMMU_TABLE_GROUP_PAGE_DIRTY);
>
> And the dirtying condition is wrong here as well. Currently it is always
> true.
>
> Honza
>

Yes. Fixed up locally. The function now looks like this (for this patch, not for
the entire series, which renames "put" to "unpin"):

static void mm_iommu_unpin(struct mm_iommu_table_group_mem_t *mem)
{
long i;
struct page *page = NULL;

if (!mem->hpas)
return;

for (i = 0; i < mem->entries; ++i) {
if (!mem->hpas[i])
continue;

page = pfn_to_page(mem->hpas[i] >> PAGE_SHIFT);
if (!page)
continue;

put_user_pages_dirty_lock(&page, 1,
mem->hpas[i] & MM_IOMMU_TABLE_GROUP_PAGE_DIRTY);

mem->hpas[i] = 0;
}
}

thanks,
--
John Hubbard
NVIDIA