Date:   Wed, 21 Jun 2023 00:05:05 +0200
From:   Michał Mirosław <emmir@...gle.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     Peter Xu <peterx@...hat.com>, David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alex Sierra <alex.sierra@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com
Subject: Re: [PATCH v18 2/5] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs

On Tue, 20 Jun 2023 at 13:16, Muhammad Usama Anjum
<usama.anjum@...labora.com> wrote:
> On 6/19/23 1:16 PM, Michał Mirosław wrote:
> > On Fri, 16 Jun 2023 at 08:57, Muhammad Usama Anjum
> > <usama.anjum@...labora.com> wrote:
> >>
> >> On 6/16/23 1:07 AM, Michał Mirosław wrote:
> >>> On Thu, 15 Jun 2023 at 17:11, Muhammad Usama Anjum
> >>> <usama.anjum@...labora.com> wrote:
> >>>> On 6/15/23 7:52 PM, Michał Mirosław wrote:
> >>>>> On Thu, 15 Jun 2023 at 15:58, Muhammad Usama Anjum
> >>>>> <usama.anjum@...labora.com> wrote:
> >>>>>> I'll send next revision now.
> >>>>>> On 6/14/23 11:00 PM, Michał Mirosław wrote:
> >>>>>>> (A quick reply to answer open questions in case they help the next version.)
> > [...]
> >>>>>>> I guess this will be reworked anyway, but I'd prefer this didn't need
> >>>>>>> custom errors etc. If we agree to decoupling the selection and GET
> >>>>>>> output, it could be:
> >>>>>>>
> >>>>>>> bool is_interesting_page(p, flags); // this one does the
> >>>>>>> required/anyof/excluded match
> >>>>>>> size_t output_range(p, start, len, flags); // this one fills the
> >>>>>>> output vector and returns how many pages were fit
> >>>>>>>
> >>>>>>> In this setup, `is_interesting_page() && (n_out = output_range()) <
> >>>>>>> n_pages` means this is the final range, no more will fit. And if
> >>>>>>> `n_out == 0` then no pages fit and no WP is needed (no other special
> >>>>>>> cases).
> >>>>>> Right now, pagemap_scan_output() performs the work of both of these two
> >>>>>> functions. The part can be broken into is_interesting_pages() and we can
> >>>>>> leave the remaining part as it is.
> >>>>>>
> >>>>>> Saying that n_out < n_pages tells us the buffer is full covers one case.
> >>>>>> But there is case of maximum pages have been found and walk needs to be
> >>>>>> aborted.
> >>>>>
> >>>>> This case is exactly what `n_out < n_pages` will cover (if scan_output
> >>>>> uses max_pages properly to limit n_out).
> >>>>> Isn't it that when the buffer is full we want to abort the scan always
> >>>>> (with WP if `n_out > 0`)?
> >>>> Wouldn't it be duplication of condition if buffer is full inside
> >>>> pagemap_scan_output() and just outside it. Inside pagemap_scan_output() we
> >>>> check if we have space before putting data inside it. I'm using this same
> >>>> condition to indicate that buffer is full.
> >>>
> >>> I'm not sure what do you mean? The buffer-full conditions would be
> >>> checked in ..scan_output() and communicated to the caller by returning
> >>> N less than `n_pages` passed in. This is exactly how e.g. read()
> >>> works: if you get less than requested you've hit the end of the file.
> >>> If the file happens to have size that is equal to the provided buffer
> >>> length, the next read() will return 0.
> >> Right now we have:
> >>
> >> pagemap_scan_output():
> >>         if (p->vec_buf_index >= p->vec_buf_len)
> >>                 return PM_SCAN_BUFFER_FULL;
> >>         if (p->found_pages == p->max_pages)
> >>                 return PM_SCAN_FOUND_MAX_PAGES;
> >
> > Why do you need to differentiate between those cases?
> >
> >> pagemap_scan_pmd_entry():
> >>         ret = pagemap_scan_output(bitmap, p, start, n_pages);
> >>         if (ret >= 0) // success
> >>                 make_UFFD_WP and flush
> >>         else
> >>                 buffer_error
> >>
> >> You are asking me to do:
> >>
> >> pagemap_scan_output():
> >>         if (p->vec_buf_index >= p->vec_buf_len)
> >>                 return 0;
> >
> >>         if (p->found_pages == p->max_pages)
> >>                 return PM_SCAN_FOUND_MAX_PAGES;
> >
> > This should be instead:
> >
> > n_pages = min(p->max_pags - p_found_pages, n_pages)
> > ...
> > return n_pages;
> You are missing the optimization here that we check for full buffer every
> time adding to user buffer. This was added to remove extra iteration of
> page walk if buffer is full already. The way you are suggesting will remove it.
>
> So you are returning remaining pages to be found now. This doesn't seem
> right. If max_pages is 520, found_pages is 0 and n_pages is 512 before
> calling pagemap_scan_output(). found_pages would become 512 after adding
> 512 pages to output buffer. But n_pages would return 8 instead of 512. You
> were saying we should return the number of pages added to the output buffer.

Ok, if we want this optimization, then i'd rework it so that we have:

bool pagemap_scan_output(..., int *n_pages)
{
   limit n_pages;
  ...
  return have_more_room_in_output;
}

The compiler should remove the pointer and memory storage for
`n_pages` when inlining the function.

Best Regards
Michał Mirosław

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