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Wed, 27 Mar 2024 06:38:43 -0700 (PDT) Message-ID: Date: Wed, 27 Mar 2024 14:38:41 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 08/14] xen/page_alloc: introduce preserved page flags macro Content-Language: en-US To: Julien Grall , Carlo Nonato Cc: Andrew Cooper , George Dunlap , Stefano Stabellini , Wei Liu , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , xen-devel@lists.xenproject.org References: <20240315105902.160047-1-carlo.nonato@minervasys.tech> <20240315105902.160047-9-carlo.nonato@minervasys.tech> <3f615066-a792-493f-ba33-7667a6557c23@xen.org> <9e0f7ff3-5457-41e4-a1e4-bf75804fb900@xen.org> <255be735-f9fe-4e39-a24e-2f6aff91acc5@suse.com> <3427132d-9458-4447-b667-d2ef3c1f0569@suse.com> <54156e94-3630-49b1-8a22-5cd4206d565f@xen.org> From: Jan Beulich Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: <54156e94-3630-49b1-8a22-5cd4206d565f@xen.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 27.03.2024 14:28, Julien Grall wrote: > Hi Carlo, > > On 27/03/2024 11:10, Carlo Nonato wrote: >> Hi guys, >> >>> Question is: How would you justify such a change? IOW I'm not convinced >>> (yet) this wants doing there. >> >> You mean in this series? >> >>> Looking at the code, the flag is originally set in >>> alloc_domheap_pages(). So I guess it would make sense to do it in >>> free_domheap_pages(). >> >> We don't hold the heap_lock there. > Regardless of the safety question (I will answer below), count_info is > not meant to be protected by heap_lock. The lock is protecting the heap > and ensure we are not corrupting them when there are concurrent call to > free_heap_pages(). > > > Is it safe to change count_info without it? > > count_info is meant to be accessed locklessly. The flag PGC_extra cannot > be set/clear concurrently because you can't allocate a page that has not > yet been freed. > > So it would be fine to use clear_bit(..., ...); Actually we hardly ever use clear_bit() on count_info. Normally we use ordinary C operators. Atomic (and otherwise lockless) updates are useful only if done like this everywhere. Jan