Re: [PATCH v10 3/6] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

[Muhammad Usama Anjum <usama.anjum@collabora.com>]

Hi Nadav, Mike, Michał,

Can you please share your thoughts at [A] below?

On 2/23/23 12:10 AM, Nadav Amit wrote:
> 
> 
>> On Feb 20, 2023, at 5:24 AM, Muhammad Usama Anjum <usama.anjum@collabora.com> wrote:
>>
>>>> +static inline int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
>>>> +                     unsigned long end, struct mm_walk *walk)
>>>> +{
>>>> +    struct pagemap_scan_private *p = walk->private;
>>>> +    struct vm_area_struct *vma = walk->vma;
>>>> +    unsigned long addr = end;
>>>> +    spinlock_t *ptl;
>>>> +    int ret = 0;
>>>> +    pte_t *pte;
>>>> +
>>>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>>>> +    ptl = pmd_trans_huge_lock(pmd, vma);
>>>> +    if (ptl) {
>>>> +        bool pmd_wt;
>>>> +
>>>> +        pmd_wt = !is_pmd_uffd_wp(*pmd);
>>>> +        /*
>>>> +         * Break huge page into small pages if operation needs to be
>>>> performed is
>>>> +         * on a portion of the huge page.
>>>> +         */
>>>> +        if (pmd_wt && IS_WP_ENGAGE_OP(p) && (end - start < HPAGE_SIZE)) {
>>>> +            spin_unlock(ptl);
>>>> +            split_huge_pmd(vma, pmd, start);
>>>> +            goto process_smaller_pages;
>>> I think that such goto's are really confusing and should be avoided. And
>>> using 'else' (could have easily prevented the need for goto). It is not the
>>> best solution though, since I think it would have been better to invert the
>>> conditions.
>> Yeah, else can be used here. But then we'll have to add a tab to all the
>> code after adding else. We have already so many tabs and very less space to
>> right code. Not sure which is better.
> 
> goto’s are usually not the right solution. You can extract things into a different
> function if you have to.
> 
> I’m not sure why IS_GET_OP(p) might be false and what’s the meaning of taking the
> lock and dropping it in such a case. I think that the code can be simplified and
> additional condition nesting can be avoided.
Lock is taken and we check if pmd has UFFD_WP set or not. In the next
version, the GET check has been removed as we have dropped WP_ENGAGE + !GET
operation. So get is always specified and condition isn't needed.

Please comment on next version if you want anything more optimized.

> 
>>>> --- a/include/uapi/linux/fs.h
>>>> +++ b/include/uapi/linux/fs.h
>>>> @@ -305,4 +305,54 @@ typedef int __bitwise __kernel_rwf_t;
>>>>  #define RWF_SUPPORTED    (RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
>>>>               RWF_APPEND)
>>>>  +/* Pagemap ioctl */
>>>> +#define PAGEMAP_SCAN    _IOWR('f', 16, struct pagemap_scan_arg)
>>>> +
>>>> +/* Bits are set in the bitmap of the page_region and masks in
>>>> pagemap_scan_args */
>>>> +#define PAGE_IS_WRITTEN        (1 << 0)
>>>> +#define PAGE_IS_FILE        (1 << 1)
>>>> +#define PAGE_IS_PRESENT        (1 << 2)
>>>> +#define PAGE_IS_SWAPPED        (1 << 3)
>>>
>>> These names are way too generic and are likely to be misused for the wrong
>>> purpose. The "_IS_" part seems confusing as well. So I think the naming
>>> needs to be fixed and some new type (using typedef) or enum should be
>>> introduced to hold these flags. I understand it is part of uapi and it is
>>> less common there, but it is not unheard of and does make things clearer.
>> Do you think PM_SCAN_PAGE_IS_* work here?
> 
> Can we lose the IS somehow?
[A] Do you think these names would work better: PM_SCAN_WRITTEN_PAGE,
PM_SCAN_FILE_PAGE, PM_SCAN_SWAP_PAGE, PM_SCAN_PRESENT_PAGE?

> 
>>
>>>
>>>
>>>> +
>>>> +/*
>>>> + * struct page_region - Page region with bitmap flags
>>>> + * @start:    Start of the region
>>>> + * @len:    Length of the region
>>>> + * bitmap:    Bits sets for the region
>>>> + */
>>>> +struct page_region {
>>>> +    __u64 start;
>>>> +    __u64 len;
>>>
>>> I presume in bytes. Would be useful to mention.
>> Length of region in pages.
> 
> Very unintuitive to me I must say. If the start is an address, I would expect
> the len to be in bytes.
The PAGEMAP_SCAN ioctl is working on page granularity level. We tell the
user if a page has certain flags are not. Keeping length in bytes doesn't
makes sense.

> 

-- 
BR,
Muhammad Usama Anjum