linux-kernel - Re: [PATCH v11 4/7] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

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Date:   Wed, 15 Mar 2023 11:55:56 -0400
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>,
        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 v11 4/7] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs

On Thu, Mar 09, 2023 at 06:57:15PM +0500, Muhammad Usama Anjum wrote:
> +	for (addr = start; !ret && addr < end; pte++, addr += PAGE_SIZE) {
> +		pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
> +
> +		is_writ = !is_pte_uffd_wp(*pte);
> +		is_file = vma->vm_file;
> +		is_pres = pte_present(*pte);
> +		is_swap = is_swap_pte(*pte);
> +
> +		pte_unmap_unlock(pte, ptl);
> +
> +		ret = pagemap_scan_output(is_writ, is_file, is_pres, is_swap,
> +					  p, addr, 1);
> +		if (ret)
> +			break;
> +
> +		if (PM_SCAN_OP_IS_WP(p) && is_writ &&
> +		    uffd_wp_range(walk->mm, vma, addr, PAGE_SIZE, true) < 0)
> +			ret = -EINVAL;
> +	}

This is not real atomic..

Taking the spinlock for eacy pte is not only overkill but wrong in
atomicity because the pte can change right after spinlock unlocked.

Unfortunately you also cannot reuse uffd_wp_range() because that's not
atomic either, my fault here.  Probably I was thinking mostly from
soft-dirty pov on batching the collect+reset.

You need to take the spin lock, collect whatever bits, set/clear whatever
bits, only until then release the spin lock.

"Not atomic" means you can have some page got dirtied but you could miss
it.  Depending on how strict you want, I think it'll break apps like CRIU
if strict atomicity needed for migrating a process.  If we want to have a
new interface anyway, IMHO we'd better do that in the strict way.

Same comment applies to the THP handling (where I cut from the context).

-- 
Peter Xu