linux-kernel - Re: [PATCH v7 3/4] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

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Message-ID: <Y9AVuF63y9UjEYcj@x1n>
Date:   Tue, 24 Jan 2023 12:30:32 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michał Mirosław <emmir@...gle.com>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.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>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.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 v7 3/4] fs/proc/task_mmu: Implement IOCTL to get and/or
 the clear info about PTEs

On Mon, Jan 23, 2023 at 05:18:13PM +0500, Muhammad Usama Anjum wrote:
> >> +	if (IS_GET_OP(p) && p->max_pages && (p->found_pages == p->max_pages))
> >> +		return -ENOSPC;
> > 
> > This is the function to test "whether the walker should walk the vma
> > specified".  This check should IIUC be meaningless because found_pages
> > doesn't boost during vma switching, while OTOH your pmd walker fn should do
> > proper check when increasing found_pages and return -ENOSPC properly when
> > the same condition met.  That should be enough, IMHO.
> This check is needed in case we want to abort the walk at once. We return
> negative value from here which aborts the walk. Returning negative value
> from pmd_entry doesn't abort the walk. So this check is needed in the
> test_walk.

Why?  What I see locally is (walk_pmd_range):

		if (ops->pmd_entry)
			err = ops->pmd_entry(pmd, addr, next, walk);
		if (err)
			break;

Thanks,

-- 
Peter Xu