linux-kernel - Re: [PATCH v18 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

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Date:   Mon, 19 Jun 2023 10:16:01 +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 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;

> pagemap_scan_pmd_entry():
>         ret = pagemap_scan_output(bitmap, p, start, n_pages);
>         if (ret > 0) // success
>                 make_UFFD_WP and flush
>         else if (ret == 0) // buffer full
>                 return PM_SCAN_BUFFER_FULL;
>         else //other errors
>                 buffer_error

And this would be:

if (ret > 0 && WP)
   WP + flush

if (ret < n_pages)
   return -ENOSPC;

> So you are asking me to go from consie code to write more lines of code. I
> would write more lines without any issue if it improves readability and
> logical sense. But I don't see here any benefit.

Please see the clarifications above.

Best Regards
Michał Mirosław