Re: [PATCH 2/2] f2fs: quota: use memalloc_nofs_{save,restore} instead of FGP_NOFS

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jul 06, 2026 at 05:59:43PM +0800, Chao Yu wrote:
> FGP_NOFS could be removed later, let's use memalloc_nofs_{save,restore}
> instead, which is recommended to be used to avoid potential deadlock
> when memory allocation in f2fs_quota_write() will call into filesystem
> interface again, e.g. .writepages, evict_inode, shrinker due to
> complicated lock race condition.

I think we need to be clear on why we need the memalloc_nofs_save()
call here.  What problem would it cause if we did call into the
filesystem to reclaim memory?

I suspect this is the wrong place to insert this call and it should be
near the lock that causes the problem.  I've attempted a rewrite of the
memalloc_nofs_save documentation; let me know what you think:

/**
 * memalloc_nofs_save - Prevent recursion into the filesystem.
 *
 * All memory allocations between calling this function and calling
 * memalloc_nofs_restore() will be prevented from calling into filesystems
 * to reclaim memory.  Clean page cache memory can still be reclaimed,
 * but (for example) inodes will not be.
 *
 * The primary reason to do this is that the caller has taken a lock
 * which would be needed by FS reclaim.  While we could theoretically
 * call into a different filesystem in this case, it can be a deep call
 * stack so it is better to avoid all filesystems.
 *
 * Filesystems often choose to incorporate a call to this function as part
 * of starting a journal transaction.  While not a lock in the normal
 * sense, it has much the same effect as nested journal transactions
 * are either prohibited or expensive.
 *
 * Also call this function if you need to allocate memory while holding
 * a file folio locked.  High order allocations (such as those requested
 * by slab) can trigger compaction which will attempt to lock the folio.
 *
 * Context: This function is safe to be used from any context.
 * Return: The saved flags to be passed to memalloc_nofs_restore.
 */