You are given a square $n \times n$. The centers of some of some $m$ of its $1\times 1$ cells are marked. It turned out that there is no convex quadrilateral with vertices at these marked points. For each positive integer $n \geq 3$, find the largest value of $m$ for which it is possible.

[i]Proposed by Oleksiy Masalitin, Fedir Yudin[/i]
1. **Understanding the Problem:**
   We need to find the largest number of marked points \( m \) in an \( n \times n \) grid such that no four marked points form a convex quadrilateral.

2. **Observation:**
   If there are two rows, each with at least two marked points, then a convex quadrilateral must exist. This is because any two points in one row and any two points in another row will form a convex quadrilateral.

3. **Case Analysis for \( m = 2n \):**
   - Suppose \( m = 2n \).
   - If we exclude the row with the most marked points, we still have at least \( 2n - n = n \) marked points in the remaining \( n-1 \) rows.
   - By the pigeonhole principle, there must be at least one row with at least two marked points.
   - Therefore, there will be at least two rows, each with at least two marked points, which guarantees the existence of a convex quadrilateral.

4. **Constructing a Configuration for \( m = 2n-1 \):**
   - Consider marking the points \((1, k)\) and \((k, 1)\) for every \( k = 1, 2, \ldots, n \).
   - This gives us \( n \) points on the first row and \( n-1 \) points on the first column (excluding the point \((1, 1)\) which is already counted).
   - Thus, we have \( n + (n-1) = 2n-1 \) marked points.
   - In this configuration, no four points can form a convex quadrilateral because any set of four points will either be collinear or form a concave shape.

5. **Conclusion:**
   - The largest value of \( m \) for which it is possible to mark the points such that no convex quadrilateral is formed is \( 2n-1 \).

The final answer is \( \boxed{ 2n-1 } \).