Hello Mark,
On 1/10/2022 3:51 PM, Charan Teja Kalla wrote:
>>> +static int shmem_fadvise_willneed(struct address_space *mapping,
>>> + pgoff_t start, pgoff_t long end)
>>> +{
>>> + XA_STATE(xas, &mapping->i_pages, start);
>>> + struct page *page;
>>> +
>>> + rcu_read_lock();
>>> + xas_for_each(&xas, page, end) {
>>> + if (!xa_is_value(page))
>>> + continue;
>>> + xas_pause(&xas);
>>> + rcu_read_unlock();
>>> +
>>> + page = shmem_read_mapping_page(mapping, xas.xa_index);
>>> + if (!IS_ERR(page))
>>> + put_page(page);
>>> +
>>> + rcu_read_lock();
>>> + if (need_resched()) {
>>> + xas_pause(&xas);
>>> + cond_resched_rcu();
>>> + }
>>> + }
>>> + rcu_read_unlock();
>>> +
>>> + return 0;
>> I have a doubt on referencing xa_index after calling xas_pause().
>> xas_pause() walks xa_index forward, so will not be the value expected
>> for the current page.
> Agree here. I should have the better test case to verify my changes.
>
>> Also, not necessary to re-call xas_pause() before cond_resched (it is
>> a no-op).
> In the event when CONFIG_DEBUG_ATOMIC_SLEEP is enabled users may still
> need to call the xas_pause(), as we are dropping the rcu lock. NO?
>
> static inline void cond_resched_rcu(void)
> {
> #if defined(CONFIG_DEBUG_ATOMIC_SLEEP) || !defined(CONFIG_PREEMPT_RCU)
> rcu_read_unlock();
> cond_resched();
> rcu_read_lock();
> #endif
> }
>
>> Would be better to check need_resched() before
>> rcu_read_lock().
> Okay, I can directly use cond_resched() if used before rcu_read_lock().
>
>> As this loop may call xas_pause() for most iterations, should consider
>> using xa_for_each() instead (I *think* - still getting up to speed
>> with XArray).
> Even the xarray documentation says that: If most entries found during a
> walk require you to call xas_pause(), the xa_for_each() iterator may be
> more appropriate.
>
> Since every value entry found in the xarray requires me to do the
> xas_pause(), I do agree that xa_for_each() is the appropriate call here.
> Will switch to this in the next spin. Waiting for further review
> comments on this patch.
I also found the below documentation:
xa_for_each() will spin if it hits a retry entry; if you intend to see
retry entries, you should use the xas_for_each() iterator instead.
Since retry entries are expected, I should be using the xas_for_each()
with the corrections you had pointed out. Isn't it?
>
